Oh my god man, just go talk to her, how the hell are you gonna get to know people if you don't talk to them, what exactly do you want his girl to do to show that she's interested in you? how exactly do you think you will get a relationship with women if you don't speak to them? if you're waiting for girls to actually approach you as a guy, you'll never get anywhere. She could be just looking at you because she noticed you staring at her, or she could be into you. Just go and ask her, better get used to these things now when you're young, you don't want to be one of those older guys who are so terrified of approaching women.
And if she rejects you or laughs with her friends, who gives a damn? you didn't do anything wrong by talking to her as long as you were being polite. I've had a couple women be rude to me when I approached them in the past, but a few days or so and I forgot about it, it happens sometimes and women who are decent would not be rude to you, so it's really insignificant even if it doesn't go your way as you'll find better women later. Literally every guy gets rejected at some point. It's like a numbers game.