Jerry P. Lightner, former teacher and head of the National Association of Biology Teachers, says this:
Providing a modest salary boost to teachers who outperform their colleagues seems like a reasonable move. Throughout most of society, employees who do outstanding work are generally rewarded with salary raises and job advancement more rapidly than mediocre workers. However, this has not been the situation for classroom teachers. With rare exceptions, teachers are locked into a rigid pattern of salary increments based on academic degrees and years of experience. Two teachers, each with a master’s degree and 7 years experience, usually receive identical salaries, even though one may outperform the other.
He has a list of suggested metrics. Some of them seem kind of hard to measure, but this is an article well worth your time.