K.H.
I think that is an unfair rule that the teachers make. After all, kids only ask certain kids for playdates - do they make that a rule too that you have to invite everyone over for a playdate? I have my son hand out invites in the playground, or usually I know the parents of the kids that are good friends of my children. Usually 9 times out of 10, the kids parents that I do know don't even RSVP, and it always leaves me in a tough position not knowing how much food to get or gift bags to prepare, so I only invite now the families that we know.