Would something from Grease work? the characters are both suposed to be high school seniors which is about 17/18. Or did you want something from a more high brow musical?
Not from a musical, but a pretty upbeat song is 'Did you ever' (I think that's what it's called), recorded by Nancy Sinatra and Lee Hazlewood back in the 60s.
Then there's 'Baby It's Cold Outside' - several couples have done that.
Or what about, 'Anything you can do, I can do better'?
Most boy/girl duets from musicals tend to be of the soppy love variety, but you could have a look at the one from Sound of Music - 'Sixteen going on seventeen'.