The Economist:

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.)

Much more on teacher content knowledge requirements, here.