how should I fund raise go and knock on peoples doors?
At least you are not saying “I can’t”, that is a powerful first step.
OK: We can 1: raise funds or 2: earn more or 3: spend less.
1: Raise funds: Organize an auction, sell raffle tickets. Sell cold water bottles in a traffic jam, or candy in the street. Bike-a-thon, swim-a-thon, run-athon. Get the students of the school involved! Explain why they should tell THEIR parents not to give to other tzedakos, and instead give to the school. And yes, you can knock on doors. (just a few ideas)
2: Earn more: Go back to school and become a nurse/OT/PT?stengrapher. Offer to babysit children when your husband is home (if you C”V don’t have one, then you are right to ask for communal funding, even though they might not be able to give it), or joint babysit your own children with others’. Sell on the internet (E-bay or other). Sell any silver/gold you may have in the house (they are at highs)(also just a few ideas).
3: Spend less: there is a website run by a frum woman called “pennyless parenting” (SJS has mentioned it). I have never been there (IIRC), but they probably have some ideas you can use. You can also cut back on meat expenses and buy more starches (just as a start).
Good luck.