Im over 18, but I play pretty young onstage. My soprano range is much stronger than my belt, so Im no stranger to looking for soprano songs with a young flair. I use The Glamorous Life from the film version of A Little Night Music, and I've had pretty good luck with that. It's Fredrika's song, if you aren't familiar with it, and she's right in the age range that I play. It is Sondheim, but I haven't had a problem with accompanists playing it.
I also use If You Haven't Got A Sweetheart from A Tree Grows in Brooklyn. It's a simple, sweet little song. Look Who's Dancing is a decent up-tempo from the same show. I've had good experiences using Young Jane's portion of The Graveyard from Jane Eyre. Jane Eyre is on a lot of overdone lists, but that song isn't available in the vocal selections so it doesn't get as much attention.
That is all that I can think of for now; though Im sure Ill come up with more suggestions later. Good luck!