IN THE film “Bad Teacher”, Cameron Diaz’s character says she entered the profession “for all the right reasons: shorter hours, summers off, no accountability”. No one is threatening to take away the first two agreeable perks, but several states are eyeing the third.

In the past, teachers were judged solely on their level of education and the number of years they had spent in the classroom—neither of which tells you whether their pupils are learning anything. But this is changing. A new report from the National Council on Teacher Quality (NCTQ), a research group, finds that most states now demand that student achievement should be a significant factor in teacher evaluations (see chart). Only Alabama, California, Idaho, Iowa, Montana, Nebraska, New Hampshire, North Dakota, Texas and Vermont have no formal policy.

The expansion of teacher evaluation is broadly good news. Work published in 2011, from Columbia and Harvard, showed that pupils assigned to better teachers are more likely to go to college and earn decent salaries, and less likely to be teenage mothers. If teachers in grades 4 to 8 are ranked according to their ability to add value (ie, teach) and those in the bottom 5% are replaced with ones of average quality, a class’s cumulative lifetime income is raised by $250,000. Bill Gates once said that if every child had mathematics teachers as good as those in the top quartile, the achievement gap between America and Asia would vanish in two years. (His lecture has been watched 1.5m times online.)

Measuring how much a pupil gains, though, is only a start. More difficult is the question of how much weight to give this measure when assessing teachers, or how to translate this into school policy through hirings, promotions and firings. About 18 states require tenure decisions to be “informed” by measures of whether a teacher is any good. About 15 states are now using teacher efficacy as—shock, horror—a factor in deciding whom to lay off. (In the past layoffs were simply by seniority, with the new hires dumped first.) And in 23 states teachers can now be sacked if their evaluations are unsatisfactory.

This new focus on merit has not affected pay or promotion, however. Good teaching is seldom rewarded with hard cash. And the first crop of new assessments looks a bit iffy. In Florida, 97% of teachers won top ratings—only slightly less than the 99.9% who were deemed perfect before the revamp. Such Mickey Mouse marking may reflect the teething pains of a new system, but is more likely to reflect union pressure against giving any teacher a bad grade, ever. Similar problems with new tests are turning up in Michigan, Tennessee and Georgia.

Some people, however, worry that the new evaluations are moving too quickly. Tim Knowles, the director of the University of Chicago’s Urban Education Institute, says: “What we are holding them accountable for is incremental improvement on blunt multiple-choice standardised tests that predict very little except how students will do on the next test.”

Another area ripe for reform is teacher training. The NCTQ describes America’s teacher-training colleges as an “industry of mediocrity”, and complains that they churn out teachers who know little about their supposed subjects and have little idea how to control an unruly class.

The countries where pupils do best, such as Singapore, Finland and South Korea, draw all their teachers from the top third of the academic pool. In America three-quarters of teacher-training colleges accept students who graduate in the bottom half of their class. Until good teachers are recognised and rewarded, pupils will be stuck with the other sort.