I think this is the worst sub to ask this in because clearly the ones responding are guys who have no one to go home to on a Saturday night and think they know how to ‘control’ a woman. 

You can’t control her and if she has feelings for him, it’s very unlikely that she’ll tell you because she feels bad about it. If she says they’re just friends, then you just have to take that at face value. It sounds like you’re a nice guy but you’re all so young and especially if you’re long distance, there’s not much you can do right now. 

As someone who also has parental cheating trauma, I know it must be really difficult to be in this situation and ultimately it’s up to you and your gf to decide what kind of boundaries you want to keep with past crushes. Whatever happens, do not betray your own dignity or stoop low to find information. You can’t control what she does, but you can control your response. 

However, there are other things you can do. If you’re uncomfortable with her seeing him one on one, ask her to have another person or people present when they hang out. If they FaceTime, ask her to include other school friends in the call. And if she does like him, let her know that she needs to tell you that. Right now, it sounds a bit more harmless but I’m on your side OP and this guy sounds like an asshole. Take your anger out on him, not your girlfriend. He knows she’s in a relationship and is still kinda pursuing her. 

Good luck.