Another non-UK person chiming in, here...

In the US (and possibly in Canada, but it's been so long since I've seen any news from home), a zero-tolerance policy was adopted after the last few rounds of school shootings. Any sort of threatening action is grounds for expulsion, and I do mean any -- see the entry for May 16th here.

I can't find any mention of such policies in the UK. Good luck, and I hope your kid stays safe...