This is the most-feared dance of all highschool girls!! Why?? Because you have to actually ASK THE GUY!!! AND wear matching shirts!!! OH NO!!!

[edit] Steps

There is NO way around it!! JUST go up to him and say "Hey so who are you going with to Sadie?
IF he says another girl's name, then GRACEFULLY say "Oh Ok have fun!!" with a smile.
If he says no one, then say "do you want to come with me?" This is easier said than done...but all guys LOVE when the girl has the confidence to just straight up ask them.
If he says "I am not going" say: "IS it because you don't want to go or because you don't have a date?"
If he says "I just don't want to go", then say, "Well if you had a date, then would you want to go?"
If he says that he does not want to come with you, say "ok your loss!" with a serious face, and then start laughing and say "Just joking, it's cool ok see you later"
Then GRACEFULLY MOVE ON!!! And ask a guy more in your league to go with you...

[edit] Tips

DO NOT try to push the guy into coming with you by tears, threats (lol), or being mean to him after he rejects you...This will just scare the others guys away from you also.

A guy anywhere from your best friend-you talking to him once or twice a week---NOT some dude you have never even spoken to
ALL the self-confidence in you
Friends--not to come with you like a gang to ask the guy, but to comfort you if he rejects and/or pump you up before you do it.