I've done stuff like this, where I'm running a session, but usually there is a teacher or other adult who is responsible for the kids hanging around. I've never had a CRB check. That was 15 or 16 year olds I think.
Never done it for more than half a day though, if what you are doing involves you being in sole charge of kids for several days, maybe things are different (but to be honest, even with sixth formers, you don't want to be in sole charge of them for long, you need a teacher or someone like that as crowd control / problem solver / generally a responsible and sensible person).
Joe