Rob Noxious
Retro Supremo⭐
The Mrs and I had a bit of trouble like this with our eldest a couple of years ago. On the third occasion we went in to the head teacher and made it clear that she could either sort it out quietly or we would publicly. Thankfully that seemed to work, she obviously didn't want a public spectacle, and the kid concerned was eventually threatened with expulsion for various issues in his very first year of school.
Our plan had been to confront and embarrass the kids parents after school in front of everybody if we hadn't got our own way; not really either of ours style but at the same time you can't start intimidating 5 year old kids without looking the villain yourself. As much as you're clearly annoyed, I strongly urge you not to involve yourself with the kids concerned.
This seems to be the way to do it. THE SEVENTIES NORTH BANK also approached it this way (see above) and that worked too.