I know how much of a shock this must be for you.

I'm not sure what the laws are in the UK, but here in the US, if the teacher overheard something like that, she would have an obligation to report it to social services authorities. All I can say is to get everything in writing from your doctor. Once they understand that this claim is unfounded, their investigation should be over. I hope your son's friend understands what his "joking" has stirred up. Hang in there.