Measuring teacher's quality

The Romanian minister of education proposed recently that teacher's salaries should be linked to their performance in teaching. But how would you measure teaching quality? Clearly not by the grades pupils receive, cause in this case everybody would start giving 10 grades (in Romania grades are from 1 to 10). Instead you should measure the (improvement in the) scores pupils receive at national standardized tests, but this is also not trivial. Here's an article about a study made in US on this topic, measuring teacher's quality. Note that in US a teacher is hired by a school, and after a few years his performance is evaluated and he can get a tenure, i.e. a permanent appointment. In Romania, to obtain a permanent position (i.e. titular) you have to pass a contest, without any proof of your real teaching capabilities.