How can I ask this girl out "Complicated"

Hi, I need help on how to ask this girl out, it may sound simple but it's not.. My sixth form collage is split between an all boys school and an all girls school. I have no lessons over the girls school so I never go over there. This girl I like comes over my school maybe once or twice a week and i rarely see her but every time I have seen her I have noticed her staring at me. I know we have alot in common as we follow each other on twitter and we like the same music, I notice she re-tweets my content alot more than she does for other people. I don't really want to message her on twitter because that's sorta weird ive been told. I could be totally wrong about her liking me but I don't know how I can speak to her as it's quite rare when I see her and when I do we are surrounded by alot of people. Should I message her or is that creepy? and what should I say..? My friends have told me she is a very shy girl. Any advice would be appreciated.

Simple, next time you see her talk to her. Invite her to lunch or some music event that you both would enjoy. I would definitely do it in person. I'm willing to bet she is just waiting for you to ask her out.

You could just message her "hey iv seen you around at school, id love to get to no you better, would you like to meet for lunch?"

what have you got to lose?

I think you have to talk to her directly. You don't lose anything you only can win. If she is really shy then you have nothing to worry about. Maybe you will be surprised because you know what people say "the calmest waters are the deepest". You can tell her about some concert or movie you both would like or just to have lunch together. Wish you luck.