The newly Republican-controlled Congress has taken up rewriting No Child Left Behind, the law that was once President George W. Bush's signature education achievement. For most people, No Child Left Behind is synonymous with standardized tests. But even though a generation of kids has now grown up under the law, it's understandable if you're a little fuzzy on what it actually does.

Here's what No Child Left Behind changed about American education, how the Obama administration has used the law to promote its own vision for schools, why many people think it's now obsolete, and what the Senate might do to revamp the whole thing.

1) What is No Child Left Behind?

No Child Left Behind, signed into law in 2002, changed the relationship between the federal government, states, and K-12 schools. The overarching goal was to make every student proficient in reading and math by the end of the 2013-14 school year, regardless of race, family income, or disability. States didn't hit the goal — more on that later. But the law is still in effect because Congress hasn't overhauled it.

No Child Left Behind, also known as NCLB, requires states to test students annually in reading and math every year between third and eighth grades, and once in high school; states also have to test students in science.

States had to decide what it meant for schools to be making progress toward the goal of universal reading and math proficiency, as measured by standardized tests, and hold schools accountable for making progress.

The law didn't only look at the progress schools made as a whole. It held them accountable for all students, including students who are from racial minorities, who have limited English proficiency, who have disabilities, or who are from economically disadvantaged backgrounds. So even if a school as a whole seemed to be doing well, it wouldn't be making enough progress to satisfy the state if a large enough group of black, low-income, or disabled students missed their targets.

If a school getting federal money for low-income students failed to hit the state's progress targets for two or more years in a row, the school had to allow students to transfer to other schools and offer tutoring. After six years of failure, schools could be turned into charter schools, taken over by the state, or have their entire staff replaced, among other penalties.

There's much more to No Child Left Behind than testing — there are sections on teacher qualifications, federal grants for districts with a high number of students learning English, and much more — but the testing and accountability provisions have always been the most debated and controversial parts of the law.

2) How did No Child Left Behind come about?

No Child Left Behind was the culmination of a major shift in federal policy. The government shifted away from just supporting local schools trying to educate low-income students to trying to ensure students were actually learning.

In 1965, the federal government began sending money to schools and school districts with a relatively high number of low-income students. The money was meant to even out a basic inequity in American education: local schools were mostly financed through property taxes. Poorer areas had a smaller property tax base than richer areas, which put poor school districts at a double disadvantage. They had poor children who were more expensive to educate, and less money to do it with.

Between 1965 and 2001, this program grew. Federal funding more than doubled between 1980 and 2000, when the federal government spent more than $7 billion per year educating low-income children. That's also when focus also began to shift toward trying to ensure that children were learning.

In 1994, Congress passed a law requiring states and districts to have the same standards for schools getting federal money for poor children and those without. That law, along with other education reform efforts at the national level, laid the groundwork for state-level education reform based on testing. Those state-level efforts would build to the passage of No Child Left Behind.

3) What was George W. Bush's role in No Child Left Behind?

When Bush was governor, Texas began using a standardized test to measure what students had learned and reward or punish schools for the result. Scores soared, improvement in reading and math that appeared to be unprecedented. On the strength of that improvement (which now seems to have been overblown), Bush made education a central part of his presidential campaign, and he promised that if he was elected to take the approach nationwide: requiring schools to test students, holding them accountable for the results, and letting students in failing schools transfer with school vouchers. (The vouchers didn't make it into the final version of the law.)

But Bush didn't pass NCLB on his own. The law united conservative Republicans like now-Speaker John Boehner, who wanted to hold schools accountable for how they were spending federal dollars, and liberal Democrats like Sen. Ted Kennedy and Rep. George Miller, who liked the promise of increased funding for education and a focus on closing the achievement gap.

No Child Left Behind was one of the last major bipartisan domestic policy achievements to pass with a huge majority. A version of NCLB passed 384-45 in the Republican-controlled House of Representatives in May 2001, and another version passed 91-8 in the Democratically controlled Senate in June 2001. But Boehner and Kennedy were stuck on resolving the differences between the House and Senate. After the 9/11 attacks, Bush pushed Congress to compromise as a proof of bipartisan governance — and they eventually did. Bush signed the law in January 2002.

4) Did No Child Left Behind work?

If you define success as getting every child able to read and do math by the 2013-14 school year, No Child Left Behind failed. By 2011, nearly half of all schools nationally weren't making adequate progress toward proficiency:

Even when states achieved high proficiency rates, they often did so by setting a very low bar — in some states, students could scored as proficient in reading who would be considered "below basic" on the National Assessment of Educational Progress, a national standardized test usually called the NAEP or the Nation's Report Card.

The evidence is mixed on whether No Child Left Behind itself improved student learning nationwide, even if all students didn't become proficient. Scores on NAEP have increased modestly in both math and reading in elementary and middle school, suggesting that students really are learning more, although those gains don't tend to persist in high school. But scores had been slowly and steadily increasing before the law was passed, too. So it's not clear if No Child Left Behind was the cause of that improvement. Even if it was, it would be difficult to untangle which part of the law was most effective in bringing it about.

No Child Left Behind succeeded in one key way: it shone a spotlight on the achievement gap between white, wealthier students and everyone else. Before the law, most states didn't release data on how poor, black, Hispanic, or disabled children performed on standardized tests at all. That's why civil rights groups, for example, are very supportive of annual standardized testing and holding schools accountable for results. The law also released a flood of data — largely in the form of test scores — about students' academic performance.

There has also been significant backlash. No Child Left Behind made standardized tests much more important. Because of the ambitious proficiency targets, an increasing number of schools were considered "failing." Some schools and districts began cheating on tests; in Atlanta, the cheating scandal led to criminal charges.

And spending more time on testing crowded out other subjects, such as art and music. A survey in 2006 found that 71 percent of school districts spent less time on other subjects than they did before the law was passed in order to focus on reading and math.

5) Did No Child Left Behind increase standardized testing?

Yes — but not necessarily in the way you think. No Child Left Behind required annual math and reading tests each year in third through eighth grades and once in high school, along with occasional science tests. That's more often than students in many states were tested in the past.

But many students now take even more tests than that. A Center for American Progress study of urban and suburban school districts found that third- through eighth-graders about 10 tests per year on average. In Ohio, which recently audited the amount of time students spend on standardized tests, students spend about 20 hours per year on test-taking.

Many of those tests are required by school districts, and not by the federal government: No Child Left Behind requires, at most, three standardized tests (language arts, math, and science) per year at a given grade level. But districts didn't decide to start testing students just for fun. No Child Left Behind attached high stakes to one-off, end-of-year state tests. So districts gave more tests to see how students are progressing and whether they'll be prepared for the big tests.

That's not the only reason standardized testing is growing, particularly lately. The Obama administration has urged states to adopt new teacher evaluation systems that factor in students' performance on tests in pay and promotion decisions. Those systems are controversial, but straightforward enough to build when teachers teach math, language arts, or science. But for teachers in other subjects — everything from history to physical education to music — some states are developing new ways to measure whether students are meeting "learning objectives." Those measurement tools don't have to be tests, but they often are. In Ohio, 26 percent of test-taking time is on tests that feed into teacher evaluation systems.

6) This is a lot of policy heavy lifting. Can we get a music break?

Yes! Well, sort of. Bush's Education Department actually hired the creators of a children's television program to write a song praising No Child Left Behind, the New York Times reported in 2002. It's really a shame that no copies appear to exist on the internet. As the New York Times wrote:

Sung in the mood of an upbeat gospel, the song bursts with praise of President Bush and of a federal law that is raising hackles in some parts of the country that worry about a too-tough emphasis on testing. The song's verses will probably not rise from the pews of any church, but they may float from telephones when the federal Department of Education puts callers on hold. We're here to thank our president, For signing this great bill, That's right! Yeah, Research shows we know the way, It's time we showed the will!

This isn't even the only song written about No Child Left Behind. Lily Eskelsen García, now the president of the National Education Association, a major teachers union, wrote a protest song in 2004 that included the lyrics "If we have to test their butts off, there'll be no child's behind left." Sadly, there doesn't seem to be video of this, either.

7) What has the Obama administration done with No Child Left Behind?

Back in 2001, few expected that No Child Left Behind would stand unchanged when the 2013-14 academic year — when all students were supposed to be proficient in reading and math — arrived. But that's exactly what happened. Congress first tried to overhaul the law in 2007, then again in 2011, 2012, and 2013. None of those attempts got very far, and so No Child Left Behind remained the law.

As the 2013-14 school year approached, the Obama administration started offering states waivers from No Child Left Behind. Those waivers meant schools could avoid the requirement that all students be proficient in reading and math, and the penalties they faced if they didn't make progress. In return, states had to promise to adopt new academic standards that prepare students for college or careers; most states, but not all, adopted the Common Core. (Texas, which is not a Common Core state, successfully got a waiver.) States also had to pledge to develop new teacher evaluation systems that include student test scores.

Forty-three states, as well as the District of Columbia, Puerto Rico, and a group of eight large school districts in California, eventually got these waivers, although two — Washington and Oklahoma — later had the waivers revoked at least briefly. In Washington, the state legislature didn't agree on a teacher evaluation system that includes student test scores, and the Education Department took the waiver away. Oklahoma lost its waiver after the state deserted the Common Core standards, but then got it back.

In Washington and the seven states that didn't originally get a waiver, No Child Left Behind is still in effect. A growing number of schools in those states are considered to be failing, and are facing the law's consequences.

8) What are some criticisms of No Child Left Behind?

No Child Left Behind, once a bipartisan policy achievement, is now broadly unpopular across the political spectrum for different reasons. Some conservatives were skeptical of the law to begin with, arguing that it was too much federal intrusion into education, historically mostly a state and local responsibility. They argue that at the very least, states should be given more leeway on how they hold schools accountable. And some think that federal regulation is the wrong way to provide accountability, and that competition among traditional public schools and charter schools — and sometimes private schools, using school vouchers — is the right way to go.

Teachers unions and some progressives, on the other hand, are critical of the focus No Child Left Behind puts on standardized testing and argue that the law never was funded fully enough to achieve its ambitious goals. (They're joined by a growing group of parents who worry about too much standardized testing, although nationally, polls have found most parents don't think their kids are tested too much.)

9) What's next for No Child Left Behind?

No Child Left Behind is now years overdue for a Congressional overhaul. The Republican Congress, particularly the newly Republican Senate, is tackling the project now with a series of hearings. Sen. Lamar Alexander, a Tennessee Republican who is the new chairman of the Senate Committee on Health, Education, Labor and Pensions, has released a draft of his idea for overhauling the law.

The biggest debate, so far, has been about the role of annual testing: should the federal government continue to require states to test every student, every year?

But even if that's settled, plenty of questions remain — particularly how much the federal government should be involved in ensuring that states are holding schools accountable, and how test scores should factor into those accountability systems. There's a divide between Republicans, who generally want to give states more flexibility, and some Democrats, who worry that states won't be vigilant about the academic success of poor and minority students without the federal government looking over their shoulder.

Alexander and Sen. Patty Murray, the leading Republican and Democrat on the Senate's education committee, have said they want a bipartisan process. But it's not clear that they'll be able to come up with a bill that could get a 60-vote majority in the Senate and avoid a presidential veto. There appears to be some Democratic as well as Republican support for reducing the number of standardized tests or their role in accountability. But Education Secretary Arne Duncan has said annual testing must be preserved. Borrowing a line from Bush, who criticized the "soft bigotry of low expectations" in his 2000 campaign, Duncan said Congress must avoid "the soft bigotry of 'It's optional.' "

If Congress fails yet again to renew the law, it could easily limp on into a third presidential administration — years after No Child Left Behind failed to meet its goal.

Correction: This article initially said Oklahoma lost its No Child Left Behind waiver. It did, but later got it back.