One powerful thing that I've done with my teens is to start with simple chord progressions but have them sing scripture to it. Or their prayers. Or lyrics to a song that goes through their heart. So many worship songs are 4 chord songs anyway that it's easy to sing snippets from a million different songs to the same chords, and it makes for a powerful and creative expression of worship.
Might want to check out some songs by Tommy Walker. "Mourning into Dancing," Only a God Like You," and "Speak to Me" come to mind. "Speak to Me" is a little slower but a great song nonetheless. Mr. Walker composes songs that are a little tougher to play musically, but with enough rehearsal (and maybe a little cheating) I'm sure the youth could get them down. The lead sheets to all his music are available on his webite for a small fee. ( http://www.tommywalker.net )
As always -- Hillsong United, Desparation Band, Lincoln Brewster, Newsboys, Chris Tomlin -- good stuff and not too hard.
May the Lord Bless your ministry!
aka Child of God