Could someone help me explain this sentense? Thanks!

Sounds like a phrase explaining techer evaluation forms. Those are the forms handed out at the end of a course where the students evaluate their teacher. The form should be worded so that students can't be seen as manipulating the techer for a better grade. Usually, these forms are filled out without the student's signature..

Thanks, littlek. but what do student motivation and other possible bias refer to?

My guess is that students may try to be more positive on their reviews of teachers in hopes that those teachers may give them a better grade than they deserve. Having the reviews be anonomous makes that bias less likely.

Does that help?

My goodness, Littlek! Did you sit at the front of the class or what? It is just as likely to prevent students who dislike their teacher (because they failed a class, for example) from getting revenge.

Val - either way, we're both cynics.

Oops. I didn't carefully read the "to get a better grade" part. Ahhh, the world is rightside up again.