How do I ask this girl out?

Im 14 in the 9th grade (high school), and theres this chick who is also in 9th grade and is 14. She is in my homeroom, that is the only cl shes in with me and like 20 other people. We don't ever talk, the only time I remember talking to her is asking her what her name is, and her replying with "Cool, you?" I really like her, she is pretty, yet I know absolutely nothing about her besides her first name. What should I do? Also, she has a boyfriend.. but she is really cute? Should I ask her for her phone number or ask her out at the end of cl when everyone leaves