Other mistake: After Professor Alcott gives back the papers, he dismisses them. A girl then comes up, and you hear him saying something to the extent of "That's the grade we use for failure. See me after class." Didn't he just end the class?
Other mistake: When the waterbed on the bunk above Paul's bed begins leaking and he gets up to hold it, you can see that the water is not actually coming from the the mattress but somewhere, like a hose, concealed by the front wood panel.