Seattle Times:

SEATTLE Public Schools is right to push for a better, more honest way of evaluating teachers, even at the risk of a strike.
Tense contract negotiations between the district and the Seattle Education Association underscore the enormous opportunity at stake. Both sides agree the current system used to judge teachers is weak and unreliable. Ineffective teachers are ignored or shuffled to other schools to become other parents’ nightmare. Excellent teachers languish in a system that has no means to recognize or reward them.
The union leadership called for a few tweaks. But the district proposed a revamped system using student growth, as measured by test scores. Supporters of the status quo have tried to downplay the other forms of appraisal that would be used. They include student growth measurements selected by the teacher, principal observations of instruction and peer reviews. Also, student input at the high-school level.

Much more an value added assessment, here.