TBH, I think what I'm going to do is to give ds 30 invites (or however many chidlren there are - hopefully the school won't have a problem with telling me the number of children in ds's class! ), with a note in each invitation apologising for not knowing the child's name, and would the parent please include in the reply their and their child's names and a contact number so that I can call them in case of a change in plan/bad weather etc.
I went to an event at the school today, scheduled to start at 10 o'clock. The parents started turning up a few mintues before the hour, we went in and were waiting in the cool lobby indoors. Headmistress turns up and tells us that the event starts at 10, would we please wait outside until then, when we would be brought in. So we're all trying to crowd into this teeny patch of shade, and several mums were heard to mutter about how fussy, pedantic and 'just-so' the school is about procedures.
Had exactly the same situation when trying to sign ds up for an after-school activity for next term: sign-up begins from 9am, but drop-off is at 8.50, and many parents have to rush off to work immediately after. Nonetheless, 'please wait outside until 9am when we will open the doors and you can come in to sign up.'
I suppose I shoudln't be surprised.