I replied to your earlier post. I'm so sorry that now others in the community are bullying you adults-- because that's what's happening now; it's adults bullying adults, basically. Not as nasty as what kids do, but equally painful, and in some ways worse because adults are subtler about it all.
I said in my earlier reply to ensure that your kids get a lot of play dates over the summer with kids who are leaving this school too -- you mentioned that other families are leaving, not just yours. Please ensure that your kids are no longer exposed to the kids of any of these families who are now bad-mouthing your family. If your kids are in any outside activities that are NOT affiliated with this school (scouting, sports, dance, whatever), work hard this summer to cultivate those friends who have nothing to do with the drama at the school.
Your pain is very, very fresh, so it's hard right now for you to see what some of us here see -- you need to let go. It's easy for us to say "get over it" and hard for you to do of course! But if people are avoiding you -- drop them. If this volunteer organization burned you because of a few members -- drop it; there are plenty of other volunteer organizations out there who would fight to get a good volunteer like you on board. If you, yourself, have other adult activities you pursue, hobbies, groups, etc, emphasize those. If people from this dreadful school are IN those groups -- find new places and things to do. You need to help your husband see, now, that fighting and fighting is only hurting your kids. They need this out of their lives and that may include you and him giving up on these families, seeing other friends and dropping these neighbors as friends, etc. He must not let his fighting instincts hurt his own kids.
Unless you live in a truly tiny town, this should not be as hard as it might seem right now. You only have so much energy and a limited amount of what I call "mental real estate." If you dwell on this, you let these adult bullies take up too much of your mental real estate; evict them.
As a Christian, I'm praying that your church does not include any of these adults who are being nasty, avoiding you, etc. If it does and you feel you need to leave it and find another, do not feel guilty; you cannot worship whole-heartedly in a place with people who are behaving this way.
As for moving, wait before you think about that. You are very emotional right now and moving house is a huge financial decision that should NEVER be made when you are upset and emotional. This can blow over once your kids are in another school and you and your husband have cut ties with the adults who are not supporting you.
The sad thing is that the bullying girl in question may indeed have legitimate mental issues that require treatment. But in any decent public school, her BEHAVIOR would have been cause for her to be disciplined or for her parents to be told to move her into a program that could deal with her. Because this was a private school run by wealthy and entitled parents, she was allowed to behave in ways that are not acceptable -- and any illness or condition is no excuse for that behavior. The parents are cheating this poor child, who will go on being allowed to behave this way "because she's got issues." In the real world outside her school, someday someone will not understand or care that she has a condition, and she will end up in serious trouble. It will be her parents' fault.