Ask to speak to the child's form tutor and make it their problem too, they will quickly also make it the head teacher's problem because no teacher wants to deal with this shit.
The head teacher is paid to take that shit, the form tutor isn't. No need to make life (more) unpleasant for someone who is probably up to their eyeballs with shit from the head teacher already...
As I’ve previously been a form tutor, I agree with you but at the same time I also know that the head teacher is more likely to listen to one of their teachers rather than a whiny parent.
The head teacher is paid to take that shit, the form tutor isn't. No need to make life (more) unpleasant for someone who is probably up to their eyeballs with shit from the head teacher already...