If you're able to be respectful and empathetic, no matter what happens or unattractive you are, the girl will most likely see you in a positive light. It's going to be difficult but you HAVE to take yourself out of your own shoes and out of your own feelings. Instead, really understand how the girl is feeling. If you can key in on how she feels and you can mirror that back to her by saying stuff like "I know this is out of the blue" or "I know you're eating lunch with your friends right now", you're gonna gain MASSIVE points. If you can show understanding, you'll gain even more points. For example, let's just say she rejects you, of course it's gonna suck, but you'll only feel the blunt of that if you're focusing on yourself. Focus on her, and without making outrageous assumptions, empathize with her and tell her you understand. For example, say she turns you down and says she's not looking for a relationship. Say "I get that, school is kind of a lot on its own and i'm sure you're busy with other things. I totally get it". THEN you leave the ball on her court without shutting the door on her face by saying "Well if your schedule ever opens up or you ever feel like talking (this last part might be too strong so calibrate accordingly), I'll be around". Then you just leave and say "have a good one!".
There is no way you can go wrong with this, you're absolutely making someone's day by showing interest in them in an authentic and respectful way. If they make a huge deal out of it, that's a reflection on the kind of person they are. You've established the kind of person you are, which is someone who genuinely respects her and who doesn't see it as a big deal, but more like a normal interaction.