To be honest I think it's pretty terrible as a group platform. No proper discussion threads and it's hard to find stuff. But people don't have to pay to host a website/forum and there's virtually no barrier to join since most people have an account on facebook.com.
The only reason I still use facebook is for these groups.
2) You make the assumption that the school does not have a private / protected group that is used for verifying group members and protecting content from non-members.
As you said the irony is that it is worse in almost every way except sign up friction. The content isn't even being spidered in any way by Google or the like so discovering the content in these groups later is also nigh impossible.
Just a travesty.
This is huge too. It's so hard to archive the data on these closed platforms - especially if you need an account ot view the data. There's a massive loss of knowledge when these platforms inevitably shutter.