Pogs
Cardboard discs, a metal slammer, and a school-wide ban by June.
You stacked them, slammed them, and kept whichever landed face up. Then every school in Britain banned them.
The ban was because it was, quite plainly, gambling. Children were losing prized pogs to older children and there were tears, then letters home.
The metal slammers were the contraband within the contraband. Heavier was better and heavier was banned first.
Cost back then
99p a tube
Goes for now
Complete sets £20–£80