Thanks for the responses. Just to clarify - there's no rule against arranging smaller parties with only a few friends, and that's what we usually do, not least because there's no way I can handle 27 kids or all the boys in the class anyway. But the teachers rule is that if you're not handing out invitations to everybody, don't do it in class because it's so obvious when somebody gets left out.

I agree that it's not the teachers job at all to be monitoring this, but I don't mind her having a say in what goes on in her classroom (like handing out invitations).

Ah well. I suspect we're worrying more about it than the classmate in question