Please, please do not ask the teacher to do anything else! She already organized 20+ invitations so that they are in the front of the folder. She has to do this for the other 20 kids when their parents decide to send out party invites, holiday party programs, and so on. Asking the schools to send out follow-ups is just unfair. I know you think it will only take a few minutes to email everyone, but teachers are already overburdened with the children's education, lesson plans, supply needs, kids with allergies and diseases and special needs, and parents complaining about real stuff and stupid stuff. So they really cannot function as the parents' social connection.
You've invited a whole bunch of kids you don't know, and their families don't know you - so you don't have contact info for each other. That's a huge problem. I absolutely agree that people are RUDE and do not RSVP, and I hate that, believe me! But by inviting the whole class, you may have inadvertently conveyed the illusion that it's a big huge party and one less RSVP doesn't matter. And every family thinks they are the only one that hasn't replied!
I would start with the parents you DO know, and mention that you are a little unsure of how to proceed since you haven't heard from 15 families, and do they have contact info for any of them. If your daughter's class has a room parent, ask her. If there is a school directory from last year, ask another parent or a rep from the parent-teacher organization. This is what volunteers are for.
Otherwise just order goody bag items from a local store, save the receipt, and take back what you cannot use. In our neighborhood, we have a lot of parents concerned about obesity and diabetes, and we often give out goodies for Halloween instead of candy. I've got a bunch of slap bracelets, erasers, spider rings, pencils, etc. in Halloween themes. Since your party is right before Halloween, maybe you could get some Halloween things, and then give out any leftovers to trick or treaters instead of candy their parents are going to throw out. So no money would be wasted.