L.A.
Oh boy.. People not RSVP'ing is one of my biggest pet peeves. I am so sorry you are having to worry about this.
I also do special events and you cannot believe the people that have the nerve to show up at events that cost a ton of money per person but they never RSVP'd..
I suggest you call people and ask them. Give them an out by saying, "I just wanted to know if your child would be attending, I am not sure if my son really got all of the invitations handed out at school."
In the future,, I suggest you send an evite.. Seems like for small parties this is the best way to get a true count of the people attending..
Please do not ask the teacher to get involved she has enough on her plate.. At our daughters old elementary, they do not allow the kids to give out invites unless all of the children in the class are invited.