I don't have an alternative that I think is workable. And that's why I'm far from convinced that teacher salaries should be tied to performance; I think the side effects of "optimization by proxy" will be severe.

As an aside, pointing out that something is bad does not put the onus on me to invent something better. The sensible conclusion is to not do anything until something good has been worked out. The alternative is to do something just for the sake of appearing to do something.