R.M.
Was the invitation issued at school or through the mail, email..some other way that the rest of the kids don't realize they are not included?
If this is a private party...at a private home then I would not have a problem with it. The problem is with the idea of a "class party"....I think End of the Year Party would have been a better way to state it.
Don't be too worried about this...as long as it isn't school sanctioned I don't see a problem