Or allow pay that is not lockstep/tied to mostly-useless degree completion. Unions fight this stuff like hell because the people in charge have gone through the current system and therefore benefit from it.

It is not that hard to do outcome-based pay over a multi-year period, but this is the main criticism. You can either track kids over a number of years and track their relative growth in each year, or you can use bargaining up-front whereby teachers can vote on a "multiplier" for each class before they know who will end up with which class.