I am positive that Dave is a figment of the character's imagination, because in the very beginning the redheaded teacher asks the character who he is talking to, when both of the characters are right there. And everyone else seems oblivious to Dave's presence, and Dave seems to have some homicidal tendencies- which in cases of schizophrenia tend to be exhibited by the phantom voices rather than the person affected with it.
 "Wasted Youth, Part 1"){kind=icon}