The only way corporations will take control of our society is if we let them. I am continually amazed by how much people let corporations and attorneys bully them. There is this pervasive attitude that they control everything and it is dangerous to fight back. If that attitude continues to prevail, then yes - that is when we will have serious problems.
I am not sure you have an accessibility case regarding a facebook group. It is accessible for individuals with disabilities, via the use of electronic aides such as screen readers. Accessibility does not mean, "I get whatever I want whenever I want." It means, "I can access this without my disability impeding that."
Not having facebook is not a disability.
I'm very, very confused by what you're saying.
But that is missing the forest for the trees - the specifics of this anecdote are one small point. Standing up for yourself is the bigger point. I could be 100% wrong about the accessibility in schools, but that doesn't change my larger wish - that we would all stand up and push back when we see problems... not just dismissively say, "well, we'd never win that fight, so don't even try."
I don't see a more accessible option. Facebook is as close to ubiquitous and universal as you can get, really. If someone cannot, not won't as the original poster said, but simply cannot access facebook, what other option is there? That means they have no smartphone, or internet access, at all. Therefore, there are no electronic options that exist. Physical paper mailers home? If they cannot afford a smartphone, what are the chances they have a permanent residence? Or, what for the parents who have no permanent residence. So that's not a solution. Next option - in person group meetings? Work, kids, health, transportation, any number of things exist that could be a barrier there. So what's the solution? That's not a flippant question, that's a real question. I don't see it.
Again, accessibility should be about ensuring everyone has the opportunity to access the goods and services in an equivalent way. Not about ensuring everyone gets to access the goods and services in the specific way they want to.
This all started with OP originally talking about not wanting a facebook. Not to sound too hateful, but part of living in society is understanding that certain personal concessions will have to be made. Like it or lump it, decisions have consequences.
Maybe I've gone too far off the path, but I think it's all related.
E-mail for instance: You need an E-mail account to sign up for Facebook, so that means E-mail's userbase is at least a superset of Facebook's. Since you need Internet for E-mail, then the user-base for a normal, Internet web site should be a superset of E-mail users. Keep looking for supersets until the effort spent reaching the next one doesn't make sense anymore.
You could try pushing back with a First Amendment complaint. Say you have a sincere and deeply held belief that using Facebook is wrong, and that forcing you to use it violates your First Amendment rights. The Supreme Court has held that beliefs do not need to be strictly religious in order to be protected. It's the same route anti-vaxxers take.
I still wouldn't hold my breath. You might win, but your children will already be in college by the time it's all sorted.