Miss Manners here. Let's discuss the R.S.V.P., shall we?
Maybe you do not know what these initials mean?
Répondez S'il Vous Plaît which is French for "Please Reply."
Apparently, many do not know this. What else but ignorance of the term would be the excuse for not picking up the phone and calling someone (or e-mailing/messaging. Those are acceptable in this modern age as well) to tell them yes, you can come, or no, my regrets, I cannot attend?
There can be no other reason, right? I mean, the person hosting the party has to plan the menu. The person planning the party has to make sure there are enough parting gifts for the children attending. How can the host make the correct arrangements without that yay/nay phone call/e-mail? The host may be sad that you cannot make it, but at least said host will know not to make a goody bag for your child.
When 30 invitations are sent, 17 to classmates, and of those 17 classmates, you receive 2 phone calls..that's bad form. If you show up unconfirmed, might your child not be given a goody bag because there are not enough? Might someone go without that slice of pizza because your numbers were not added in the planning?
Please, kind reader. The next time you receive an invitation, Répondez S'il Vous Plaît.