One that comes to mind is "The Fantasticks" by Tom Jones and Harvey Schmidt.
It has a small cast, the singing is not overly difficult, a simple set, and
best of all, it's not too long. Check it out.
Also, if you go to MTI's website (http://www.mtishows.com), you can do a
search for musicals that are especially suited to high schools. You can also
get info on "The Fantasticks" there.