If that happened, there’d be a period of time where they’d get away with it. As Todd Ingram showed, it’s not hard to cheat on your girlfriend with your band mate. However, considering it’s high school they’d probably not have too many places to fool around without being noticed. Scott lost his virginity in a backseat of a car, so it’d probably raise suspicion if he kept riding with Lisa at night. My headcanon is that he’s probably good with his fingers and most of their illicit activities are hand stuff (it’s still high school).
Anyway, secrets like that are hard to keep in high school so Kim would probably find out. High school drama ensues, Kim and her friend gets mad, their band breaks up and it’s possible Scott and Lisa continue their relationship. Also, their geography class gets really awkward for the rest of the year.