Prom is coming...What should I do?

I have a secret crush on this guy and he is a senior. We were never close but we knew each other. We will be having our annual js-prom and the juniors and seniors will be together at the event. I really want to have a dance with him but it kind of makes me embarred. He also has a girlfriend and I don't want to make a scene but this might be the last time I will see him. I am not taking him away as a date or anything, I just want one friendly dance with me so I can say I had a courage to ask someone to dance when I am in high-school. What should I do?