This comment from Dana Goldstein is directly on point:
I'm often asked what one education reform I think would make the biggest impact on American students' achievement. I don't like this question. The real answer--vastly decrease poverty--isn't going to happen anytime soonIt may not be in the cards, but poverty and the knock-on effects of poverty are hard to ignore in the context of United States education policy. It simply connects to too many different pieces. Neighborhood funding models for schools mean poor neighborhoods end up with under-resourced schools. High rates of incarceration increase issues like single parent families and foster care families. Lack of trust in social institutions makes short term planning much more rational.
All of these things end up impacting the overall quality of education. Any inefficiencies due to a teacher's union preventing teacher recruitment are likely to be a rounding error compared to the main effects that we are estimating here. After all, when there are enough problems with home life (stress, food insufficency, lack of child care, etc . . .) there is only so much that a teacher can do for the 30 students in his/her class.