Is it prohibited slander to give a teacher a poor evaluation? R. Dr. Asher Meir discusses this in a Jewish Ethicist column (link). Basing himself on the analysis of R. Dr. Aaron Levine in his book, Moral Issue of the Marketplace in Jewish Law, R. Meir responds:
[T]he answer to your question depends on what kinds of questions the survey asks and to what use the answers are put... For the individual student, the most important thing is to fill out the questionnaire honestly and not have any vindictive intentions when giving a negative evaluation. If the use of the questionnaire is improper, I think that there is seldom any benefit to an individual student withdrawing their participation, but it is appropriate to complain to the administration if you think that the results are being used inappropriately.