Question regarding women....?

alright well i go to an all guys school... (ya ino it sucks, not literally) and i work at a foodstore as a cashier on the weekends. theres this girl there that i work with. i try to talk with her as much as possible and the conversations are ok best. i think she likes me by the way she acts. im not gay or anything, its just wierd talking to girls, since im surrounded by guys all the time at school.(even the teachers are like all guys, and of course the priests/brothers are guys) and we got a semi formal at the end of the month and i wanted to bring the certain girl to it. anyone have any ideas on how i can ask for her number? if i can get that i think i can work my way into asking her to go to the dance with me.