How to Ask this Girl to the Dance?

I'm a Sophomore at a rather small high school, and the Freshmen-Sophomore semiformal dance is in a month. There's this new girl at my school that I have once class with, which is the first class of the day. She has a twin brother in that class as well, but their schedule is full of electives because they came in the second semester. The only time I really get to talk to them is in the class we have together and if I see them in the halls or something. Yesterday, I was talking with this girl through the whole class, longer than ever before, and it was great. I can tell that she likes me as a friend at least, but I still do not know her very well. However, I was thinking, although it might be a spur of the moment kind of thing, about asking her to the semi. How can I ask her? I didn't really want to go over the top, so I'm looking for simple methods and needing to do it in school. Thanks for your help!