Brian Davison had to go to court to pry loose state data about student performance in the public schools. Now teachers are going to court to keep him from sharing it.

Davison, a Loudoun County, Virginia resident, sought data on student growth percentiles (SGPs), a measure of how well students progress from year to year. The Department of Education didn't want to release it, and the Virginia Education Association didn't want it to. But last January Richmond Circuit Court Judge Melvin Hughes rejected the argument that releasing the data would necessarily compromise student privacy. He ordered the data released, with identifying information redacted.

Teachers don't like that one bit, because data about student progress can be used to measure teacher performance. And if there is one thing the public education system does not like, it's competition—either with private schools (through school choice), or with alternative public schools (through charter schools) or among teachers themselves (through merit pay). The education establishment is dedicated to the proposition that all mentors are created equal, and any suggestion that some might be better than others is anathema.

That's why the Los Angeles Times provoked widespread fury by publishing the ratings for 11,500 teachers in the Los Angeles district. On its interactive website you can look up teachers by name. You also can look at the performance of specific schools. The head of the L.A. teachers' union was "outraged." The president of the National Education Association said "it is unconscionable to evaluate teachers in the public square." The quality of the evaluations themselves became a subject of debate. Some people even blamed the newspaper when a teacher committed suicide.

But John Butcher, a Richmond resident who writes extensively about the public schools on his blog, CrankysBlog, told the Washington Post that, using Virginia's data, "he found a teacher whose students on average ranked in the bottom 1 percent in student growth percentiles. 'Any kid who is stuck with one of those teachers is going to have a problem,' he said. 'Why would you want to hide that information from parents?'"

The answer, at least for public consumption, is that parents are too dumb to be trusted with it. "I feel that the parent might… get the wrong impression of a teacher if they don't completely understand the data," said Loudoun School Board member Debbie Rose.

There probably are other reasons. In one post on his blog, Butcher points out that despite poor student scores, according to the Richmond school system's own reports, "only 0.72 percent of the items in Richmond teachers' evaluations showed some aspect of failure to meet or exceed expectations in 2011."

Butcher says this is "absurd in the abstract; in light of the available data it is baldly mendacious." The SGP data he was able to obtain show that while "Richmond has some outstanding math teachers," they are outweighed by under-performers. The picture in reading is more bleak: 109 out of 205 reading teachers are below the state mean, he finds; 52 are more than one standard deviation below the mean, and 16 are more than two standard deviations below the mean. (A standard deviation is the average distance from the average; if the average is 50 and the standard deviation is 2, a score of 46 is two standard deviations below the mean.)

All of that is a complicated way to say that many Richmond teachers may be doing a much worse job than they are getting credit for. Granted, the SGP is an imperfect metric, and the state's Department of Education has replaced it with a new one based on "value tables." Still, it's possible to adjust for flaws in a given metric. You can make it weigh less for teachers with less experience, or take a three-year-average to adjust for one year when a class has several slow learners. But at least using a data-based measurement has some connection to empirical evidence— unlike, say, a subjective classroom observation.

So it's not surprising that the VEA and several local teachers' guilds went to court a few days ago seeking injunctions against the Virginia Department of Education, Davison, and Butcher. The guilds want the court to stop VDOE from releasing any more of the data Davison and Butcher have requested. And they want the courts to stop Davison and Butcher from publishing any of the data VDOE has already handed over. Releasing such information, the guilds say, could "be used to irreparably harm (teachers) in the profession."

Not releasing it, however, could irreparably harm students, along with parents and taxpayers who would be kept in the dark about which teachers are doing a great job—and which ones aren't.

This column originally appeared at the Richmond Times-Dispatch