wow - this is a tough one.. I have three boys, same class and all are invited to classmates parties.. I have no personal experience, however if I did I experience this. I would explain that everyone has their own friends, and let the boy who was invited go to the party.

My boys play on different soccer teams this year (2/1 split due to different skill level) , anyway one was invited to a party of a teammate- absolutley refused to go, because his brother's weren't invited.. (his brother's didn't even know the kid that well.. My guys are thick as thieves.. Had he wanted to go I would have let him..

good luck..

Lisa mom to Melanie(28)
Guillermo,Jonathan,William(10.9.01 @34 weeks)
grandma to Brianna(5)