Just last year, a Morton, Ill., school board member protested the purchase of a science textbook that favored an “Old Earth” origin story. Conservative parents in suburban Chicago opposed a day-long seminar intended to foster discussion about the persistence of racial division in American life. And a Republican lawmaker in Arkansas proposed a ban on teaching the late Howard Zinn’s popular left-leaning interpretation of American history, “A People’s History of the United States,” in public classrooms.

These curriculum controversies are not new. At their core is a debate over power and hierarchy in U.S. society. Those individuals and viewpoints that are valued in school curriculums have a decided advantage when it comes to making claims of moral authority. If American children, for example, grow up learning that evolutionary biology is the key to understanding human origins, creationist Americans will have a much more difficult time getting a hearing for their views and will thus lack moral authority in the important realm of science. Yet while curriculum battles shape and are shaped by the nation’s larger cultural wars, they also threaten to undermine a pillar of American democracy that should concern both sides: public education.

Early challenges to public schools came from economic and religious concerns. The Protestant elite who set up the common school system in the 19th century believed schools provided training and acculturation for the poor and working class. The working class decried the invasiveness of compulsory education, and in manufacturing towns such as Beverly, Mass., voted to discontinue the high school in 1860. Catholics also saw the public schools as an attempt to indoctrinate children with Protestant beliefs. They began to build their own network of parochial schools — building institutions to challenge the cultural authority of Protestantism.

In the 20th century, religion continued to play a central role in debates about public schools, but the lines of conflict had changed. As the public square slowly but surely grew more secular, Protestant Christianity lost its grip as the sole arbiter of moral authority in American life.

In this new America, instead of working-class Catholics resisting Protestant control, religious conservatives began protesting the hold that secular liberals had on the school curriculum, epitomized by the fact that an increasing number of schools quit requiring mandatory Bible reading. A few schools even began teaching Darwin’s evolutionary science instead of creationism, a trend that sparked, famously, the 1925 Scopes Monkey Trial in Dayton, Tenn.

The culture war between religious conservatives and secular liberal educators intensified during the 1960s. Cosmopolitan-minded educators believed it to be their job to solidify civil rights gains by making anti-racism manifest in the curriculum. To do so, they took it upon themselves to overturn curriculum materials that, as one left-leaning educator put it, “tend to perpetuate images of white, middle-class, suburban families living in traditional bliss.” As private Christian schools in the South enrolled a growing number of white students whose parents did not want them attending integrated schools, the educational link between secularization and civil rights intensified.

But above and beyond resistance to civil rights, by the 1970s, religious conservatives had valid reason to believe the nation’s public schools no longer represented their moral vision.

The Supreme Court enshrined secularism in the schools with landmark cases, most notably the 1962 Engel v. Vitale decision that declared school prayer unconstitutional. Pedagogical trends were just as distressing. In social studies, students were increasingly challenged to clarify their own values, independent of those instilled by their parents and churches. In science, evolution was taught more than ever. And in health classes, honest discussions of sex replaced moral exhortations.

Religious conservatives fought back to reclaim their moral authority in public schools.

School board hearings became political battlegrounds in the 1970s. One parent protested that a guidance counselor regularly visited her child’s classroom to probe students about whether they believed in God. Another was outraged over the curriculum, which he described as “a very subtle way of teaching our children genocide, homosexuality, euthanasia.”

Legislation was the next step. Sen. Orrin G. Hatch (R-Utah) responded with a “Protection of Pupil Rights” amendment to the 1978 reauthorization of the Elementary and Secondary Education Act. Grounded in the “family values” rationale, the Hatch Amendment allowed parental supervision of educational materials produced by the federal government. It also stipulated that parents had the right to shield their children from any such materials they found objectionable. They, not President Jimmy Carter’s newly created Department of Education, had the final say over what their children learned.

Such early gains emboldened religious conservatives and reinforced the belief that they could turn back the tides of a secular and liberal curriculum and reclaim the days of school prayer. Overturning liberal reforms seemed possible.

Yet since the 1970s, religious conservatives have witnessed one defeat after another in the realm of educational politics. President Ronald Reagan failed to pass his promised school prayer amendment. Efforts by some states and districts to teach creationism, or what later came to be called “intelligent design,” were defeated in the courts time and again. Even in periods when conservatives controlled the White House and Congress, the school curriculum remained secular and liberal.

Why? Because the left has largely won the culture wars that pitted secular and cosmopolitan liberals against religious and traditionalist conservatives in a pitched struggle over what it means to be an American.

This victory has changed the nature and purpose of the public-school curriculum. But this seeming victory by the left has had a damaging unintended consequence: Defeat has led many religious conservatives, like 19th-century Catholics, to abandon the public schools.

Networks of private Christian schools have expanded across the country since the 1960s, and the ranks of the home-schooled have grown from 250,000 in 1985 to well over 1 million by 1999. An estimated 90 percent of those who home-school their children are conservative white evangelicals.

But whereas the 19th-century public schools were too scattered to be weakened by the Catholic exodus, the conservative abandonment and the ongoing curriculum wars put our schools at risk. By weakening public trust in the schools, such conservative activism fosters the notion that schools are in crisis, ultimately making them vulnerable to a variety of education reformers who are ultimately more threatening to American public education than religious conservative activists.

Powerful people from across the ideological spectrum — from Bill Gates to Betsy DeVos — favor standardization, testing, accountability, union busting and charters. These reforms strip power and autonomy from teachers, dull creativity in the classroom and threaten to undermine much-needed critical thinking and analytical skills. They are the true threats to public education, and all parents should oppose them no matter their political ideology.