At this point, there are ten states without teacher contracts protected by a teachers’ union. And of these states, nine have test scores far beneath the national average, while one, Virginia, has average scores.
Of course, there’s not necessarily a correlation between test scores and job security–but it’s certainly quite a “coincidence” that the scores would turn out this way.
A Washington Post article provides the statistics, pointing out that there are certainly many factors that contribute to the low test scores, but at the very least, the scores show that teachers’ unions are not to blame for some of the lowest scores in the nation, since these states don’t have unions!
I would conclude: just because popular discourse blames teachers’ unions for poor school performance doesn’t make it so.