Legal Concerns

I guess when it comes to legal concerns for my future as an educator I think a lot about what my legal responsibilities will be and whether or not these responsibilities will be reasonable. Specifically, I am concerned with how they might begin evaluating a teacher's performance based on test scores and start basing pay, and job security on these evaluations. How innaccurate could these test scores reflect how well one fulfills their job as teacher and instructor? It is kind of a complicated topic but it is one that concerns me.