Like, is she in high school? Is she bullied? She's friends with the male MC by then, right?
Maybe one of the mean girls knows the guy and likes him, or sees them together and thinks Prudence has a boyfriend while she doesn't, or something, and spreads a rumor that he knocked her up or something, and then her parents hear and think it's true, and she gets super ashamed and no one believes her?
literally straight off the top of my head.