"This survey clearly shows that parents are not clamouring to set up free schools, have no issue with schools being accountable to the community through democratically elected local authorities, and absolutely reject the premise of their children's education being handed over to private companies. Free schools are not wanted or needed. They are divisive and unaccountable. It is time the government stopped playing with the educational future of this country based on nothing more than the fact they can."
The NUT is not directly concerned with the wellbeing of school children, but rather the pay and conditions of its members. It sees a threat from schools that are more independent, because they may not be so vulnerable to union coercion, and these schools may draw away children from the more talented end of the spectrum, hence reducing the average at the local authority schools. To the coercive collectivists, found in great abundance in the NUT, the duty of a smart kid is to go and stay in a shit school, to raise the average.
Whatever the fruits of the government's policy (and I should say, as a libertarian my choice would be to smash the state-run system into a thousand pieces, and then to take those pieces and grind them into dust), the NUT's critique must be taken with a generous pinch of salt. Just because it pisses off that bunch of recalcitrant syndicalists, doesn't make it bad, more likely the opposite.
Update: You can hear the NUT making a fool of herself on the Radio 4 news here.