OP here. My friends and I checked him out and he has a history of doing the same thing to girls in his own grade (he's a year below me). He bugs them online and invites himself into their lunch group, as well as watching from behind poles. He isn't very physically threatening, so no one sees him as a threat, but more of an annoyance. And very creepy. I'm not rude to him, but I don't want to encourage the behaviour so I just ignore it. When he comes over to our group we start talking loudly about bras or tampons and he wanders away soon enough.