@Loci It may be that Dave odesn't even know he is even dead. He suspects he is stll alive, therefore he turns in the paper. It may also be that the teachers do not check the grades either, for if you read the principals biographies on the students, he tells them to change grades and all. So therefore, if the principal doesn't really care, maybe so do the teachers. Also, I have came upon a recent discovery. Dave is underground in the begging of the game, when they tell you to explore, I walked outside and there is a girl running around trying to become fit. There you will found a mound of dirt, this I expect would be Dave under there. Xerxes may be the killer. He has been known for joining such groups, maybe he had come upon a killer group. It may have something to do with that. He is also sik of everyone if you happened to open the box that you open by covering all green colors.
Heres something that can support SpinelessCoward's theory, in the beggining Dave is hanging out with the "cool kids" and they don't seem to see him, also the teacher exclaims, "WHO ARE YOU TALKING TO!"
 "Utopian Mining"){kind=icon}
 "Halo Galaxies 2"){kind=icon}
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 "Platform Racing 2"){kind=icon}
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 "William and Sly"){kind=icon}
 "Wasted Youth, Part 1"){kind=icon}
 "Civilizations Wars"){kind=icon}