NAH but I think you committed slight social faux pas. First, even with the teachers handing putting the invitations, potentially discreetly, in bookbags, it would probably be better to mail them. Second, I personally feel like inviting half the class is quite large for that sort of thing. In my childhood it was generally like a small 8 or so person sleepover type party, or a big entire class type bowling party, but generally wouldn't be a party that large without inviting everyone. The more people you invite, the more exclusionary it feels to not be invited. However, those are just my personal opinions and I wouldn't say you were an asshole in this situation.