You'd need to be a bit wary about approaching schools, for 2 reasons:
1. You'd need to check that the proposed venue (if a pub) was OK about under-18s being admitted. I've seen a competent 16-year-old flautist who was accompanied by her mother being asked to leave a pub session.
2. Some parents might send younger children in the hope or belief that it was like sending them to Scouts or swimming lessons - in other words, you'd be in charge of the kids. You'd need to work out who was "in loco parentis" (supposing a child is ill, or leaves the room and perhaps sets off home early, etc); or else you insist that a parent should accompany the youngster, which would then change the whole balance and atmosphere of the session. You might even find that all the adults had to have CRB checks....

If it were me, I wouldn't suggest having under-18s at a session.

Marje