Its been a while but I have taught Stained glass to high schoolers but not
middle schoolers. You can avoid the lead issue by having them do copper foil
technique with lead free solder. I would say if you can trust them with a
linoleum cutter then you could trust them to cut and grind glass. If they
use running pliers or the other kind to break the glass then they should be
allright. You should have good ventilation for soldering.
I have also taught high schoolers how to blow glass. That gets them psyched