I don't think he blackmailed her, it was probably for drugs. As for why he wouldn't be allowed in school is that he has bad grades, he harasses people in school, probably rape (but maybe not, who knows) and the biggest one of them is that he harasses Priscilla, who should never be harassed, you know? You could say that she just didn't tell her parents about it, but it's just lazy writing. For example, Darnell should've had his ass kicked out of school as soon as he harassed her, without even needing Priscilla to tell her parents about it. Because you know how fucked principal would be if he didn't heavily punish Darnell for it, and Priscilla told her parents about it? Really fucked.