Depends on what you are looking for. Are you looking for some place where a single diver would hook up with a buch of other kids and go somewhere, or looking for a place where a group of youth could head off somewhere together?
Either way I would check with international field studies. There is a camp in the bahamas called forfar field station its on Andros island. They do a lot of stuff besides dive there and also have a sailboat option where you sail around to the Eluthras or other islands that aren't as inhabited. Also another place in the bahamas is on San Salvador. I haven't been to that one so I don't know all the details. I think both of these places take both groups and individuals. However their main purpose is marine science education, with a little diving thrown in on the side.
Aggressor runs a teen week (13-18 I think) on several of their boats, that would be all diving.
SeaCamp is another option, Fl Keys, SanDiego, and HI.
BoyScouts opporate Sea Base also in the Keys but both have a science focus as well.
you might also PM Archman. He is working at a place for teens in the bahamas this summer, and might have some additional info.