What's easier from the teacher/teachers representative, use 1 widely distributed app, or download 100 different messaging programs/interfaces and make sure every student knows the proper channels to contact every other student?
There will always be someone unhappy, let's try to keep that from being the person who has to do this dance day in and out for the rest of their underfunded lives?
Fine, use one app but make it the Free Software one, then.
Regardless, it is completely unreasonable to dictate that people consent to onerous third-party corporate terms of service in order to access government services, especially ones enforced by truancy laws!
There will always be someone unhappy, but let's make them unhappy with mild annoyances and not serious privacy violations. I'd rather prefer the Whatsapp users to be unhappy than the Signal users. In a long run it's doing them a favor.
It's like four or five. We did this back in the late 90s. Some people used AIM. Some used Yahoo Messenger. Some used ICQ. And some used MSN. We survived.
It was such a common thing that multi-protocol clients like Trillian were popular.