Uggh...my friends little girl just had chicken pox- a bad case. Poor baby had fever, and a bad break out of them, and because she's only 16 months, she couldn't help herself to scratch, so they had issues with that too.
If it were me, I wouldnt want the child around with it, but they are in the same class, so there is a huge chance hes already been exposed, so really it may not matter.
Why would they want to bring a kid to a party who has chicken pox anyway?