People do not respond. They just don't. When we stop expecting them to do that we'll all be a lot happier.
Sit down and call them. Even the ones who say they will be coming probably won't come.
It would just be a lot easier if everyone just planned a simple party and invited everyone then let them know they could just come if they wanted to and didn't need to respond.
If you have extra cake then you get to send some home with the guests that come or you get to eat some extra cake that week.
People just don't see the invitations half the time. You have no idea if they've even seen it.
Get a phone list from the teacher and call everyone. That way you can talk to the parents voice to voice. Still, some will forget and not even show up then.