Need Girl Advice... What should I do in this situation?

So.. I like this girl at my school but she has a boyfriend... They just started going out like 5 days ago and before she started going out with him I heard her telling her friend she didn't want to go out with him because she thought he would get no where in life. Well there is a farewell dance coming up and I want to know if I should ask her to that, wait till they break up and ask her during summer or wait till next year? Btw school is over in 7 days... Now why I think she likes me.. Whenever I am walking down the hallway behind her she looks back a couple of times... Now she stops and walks behind me.... Also in cl she has her knees my direction and is either looking at me or in my direction.. Please give me advice on what to do.