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I do wonder if by banning it in schools it will get less and less common till it disappears. I suppose you predictably think that's nonsense.
But pedantry aside, its banned for the people who used to play it most and enjoy it, at the place they did just that.
How could I be so clueless?
“What exactly are you so happy about?' Harry asked her.'Oh Harry, don't you see?' Hermione breathed. 'If she could have done one thing to make absolutely sure that every single person in this school will read your interview, it was banning it!”
The ban is generally lifted or just not enforced at all after a year or two.
For starters the playground monitors have more important stuff to do than remember the list of banned activities and try to enforce them.
As for "the law" - from a 2019 petition to make conkers legal again:
There's no law or government policy banning children from playing conkers, so we're not sure exactly what you'd like the Government or Parliament to do.
EDIT: And now I feel guilty for doing so.
Believe it or not the world has come on a long way in 16 years.