Corrected entry: Towards the end of the movie Allison reveals that she's attending the detention simply because she has nothing better to do. This was probably done to emphasize her being a loser with no life, but it's ludicrous in realistic terms. Vernon would have surely had a list of the students scheduled for detention, and would have immediately realized that one extra student was there who had no business being there. And as far as I know there is no High School that would allow a student to sit through a detention session just for the fun of it.
Sereenie
11th May 2004
Correction: Allison also states she is a compulsive liar.][Vernon didn't know her name either, it's possible that she wasn't supposed to be there and he didn't care.
Sereenie