They are going to know they were not invited anyway.I would guess the school just doesn't want the kids who never get invited to anything to have to publicly go through that every time a classmate's birthday party comes around.
And if there is truly a child in the class that NEVER gets invited to anything, he certainly isn't going to feel good when he, himself, invites everyone in the class and none show up.
Or even worse, he is invited, along with the whole class, due to the rule, and everyone at the party ignores him because they didn't want him there to begin with.
So the rule is NOT going to prevent his hurt feelings.
No more burdensome that asking them at school.Is it really so burdensome to have your kid call up his friends?
Nothing wrong with doing it either way.