Even if students are colluding to give poor ratings to teachers they dislike, it won't significantly alter the relative rankings of the teachers - the better the teacher, the fewer students that are willing to systematically exaggerate the teacher's weaknesses, and the best teachers (the ones who make learning fun) won't have their scores hurt at all. Students colluding will make it hard to set a threshold for "good enough" to not fire, but even if the students catch on, it should still be a good tool for identifying the best teachers.