In many school districts, basing a teacher's pay on student performance is like paying the auto worker who installs the last part on a finished car based on the quality of the entire car that rolls off the line. If the car is a piece of junk by the time it reaches his position on the assembly line, it's unrealistic to expect that he'll be able to produce a good finished product.
Wife and I watched the Michelle Rhee PBS show last night; apparent there was cheating with test taking.