OP, perhaps you should drop a word to the instructor about this. He needs to remind the students that the purpose of the critique needs to focus (I'm assuming, never been an artist!) on the artists work and the purpose behind the exercise, and not the subject matter the artist chose for the work.

Otherwise, anyone who draws anything remotely scary or controversial, whether it's a snake, spider, firearm, or the boogie-man, is not going to get an honest assessment.