My experience is that, no matter what you do, a lot of people just don't reply because they are too focused on themselves or because they think it doesn't really matter because you're inviting the whole class anyway. We see on Mamapedia all the time that people invite everyone hoping that half won't come, and those invited get sick of an invitation or a party every week or two. So you can't win.
Evites are easier to follow up on though, so I guess I'd go that route. People like to get "real" mail but then they put the invitation aside and figure they can reply later when they're at the computer. Then someone puts the mail or the soccer schedule on top of the cute envelope, and that's the end of it. Finally you contact them again saying "I really have to know" and they either reply or feel guilty so they put it off.
We just never invited the whole class because half the kids weren't our son's friends and the parents didn't know us, so there was no real motivation (unfortunately) to be all that polite or accommodating. You see it in the lack of thank you notes too. That's another reason we just kept things small, so it was more intimate, easier to keep track of, and far less expensive that a party for 20.
Good luck.