My nieces and nephews had some school announcements _only_ disseminated on facebook. I had to call the school to change their process, and even then it took a while for them to adapt.
This kind of thing absolutely needs to end. Especially for public schools. Requiring you do business with a specific commercial entity just so you can get a message from your government definitely crosses a line, and you were 100% right to advocate that they change.
You are severely downplaying the network effect.