Corrected entry: When Mr. Gierasch finds Charlie's cheat sheet on the floor he apparently has no idea who it belongs to, but he could have simply compared the handwriting on it to the handwriting on all of the students' exams - whichever exam matched the handwriting on the cheat sheet would have immediately exposed the cheater (Charlie). Mr. Gierasch would certainly have been intelligent enough to think of doing that as opposed to the way the sequence plays out only to benefit the movie's plot.
LorgSkyegon
31st Oct 2018
Correction: We see during the student meeting that Gierasch told them it was block printed. Charlie intentionally disguised his writing in case he was caught.
LorgSkyegon