Liberals often complain that those of us who support school choice are just interested in smashing the teacher's unions. And to some extent, they have a point. To be clear, I do not object to the teacher's unions because they have a union. I object to the teacher's unions because teachers are among the competing interests that run low-income school districts for the benefit of the various interest groups, rather than the children. The union merely gives them more power to move value from children to teachers.
I do not say that they are malicious, though certainly in many cases the union clearly recognizes that they are benefitting their members at the expense of the children. But more of it is that the entrenched institutional arrangements, many of them enshrined in union contracts, are extraordinarily impervious to change. When an entire system has grown up around union arrangements, tweaking any substantial part of it threatens to throw the whole system into disarray.
Unions also give teachers power to resist changes that make their jobs less fun. I think the teachers genuinely believe that these changes are bad; but I also think that they strenuously resist learning anything to the contrary. There is really good evidence for the benefits of direct instruction in teaching disadvantaged children. But direct instruction moves the teacher into being more of a technician and less of a creative professional. Ian Ayers talks about this in Supercrunchers, giving the example of bank loan officers, which used to be a skilled, prestigious jobs, and are now almost a clerical role. Doctors and teachers are resisting an attempt to do similar things to their jobs through, respectively, evidence based medicine and direct instruction.
But it's more than that. In New York, the principal's union resisted an attempt to attract the system's top principals to failing schools by giving them a substantial bonus payment in the tens of thousands of dollars. The union vetoed this because the extra pay wouldn't accrue pension. Huh? It was entirely voluntary, the system couldn't afford pension payments, and the principals would have gotten an extra $25 grand or so. But no dice. Any change threatens the union, because it puts the delicate balance of power between all the competing interest groups in play.
Liberals rejoinder that it isn't the unions--it's the funding/poor kids/infrastructure/class size/textbooks. This sort of thing is hard to disprove conclusively, of course. But here's a data point: New Orleans smashes it's teachers union; test scores rise dramatically, even though it's still ministering to poor kids testing substantially below grade level.