You should usually trust your gut, and in this case, you're almost definitely correct that this other guy is hitting on your girlfriend. Get used to it - any woman you date is going to be getting hit on continuously.

The only real issue is whether your girlfriend is a) so naive that she doesn't realize she's being hit on, or b) knows exactly what's going on but just likes the attention. Not sure how old she is, but if she's over 16 or so she's figured this out so, it's almost definitely b). b) is a maturity issue, most women eventually grow out of it, but this is a high school girl we're talking about here.

So, in all likelihood she just likes getting extra attention from guys. I'd give her the benefit of the doubt and try to talk it over with her. Say hey just in case you're not sure, this other guy is definitely hitting on you, and I am not cool with you hanging out with this guy alone without me. Kind of gauge her reaction from there. If you have a good talk about it, then great! You can trust her. If she gets angry or defensive, then you cannot trust this person and you should break up with her and find someone you are on the same page with.