theleveller,
You bring up an excellent point. In the US at least powerful teachers' unions protect the worst. Everytime it is suggested that teachers be themselves tested to be sure they are up to date on new methods as well as material, the union fights it. Everytime the suggestion is made that there be rewards for good teachers measured in for example how many students graduate, the unions fight it. Tenure is another problem. What other job guarantees that you can stay no matter what your performance?

Why not reward the best teachers and set goals for those not performing to standard and hold them to it?