You can. It's almost quite literally an effort of 30 - 60 min per day or week or month, depending on how skilled you are at keeping things convenient, meaning self-hosting services and maintaining them.
> Did you organize meetings with parents? In coop with teachers?
At least where we are, schools have been told not to touch WhatsApp at all, and use another (more GDPR-compliant) messaging product; despite this there is typically an unofficial WhatsApp group for each class, but no teachers are members.
> WhatsApp means obedience and submission
I don't disagree, but I don't think you'll convince many parents to stop using WhatsApp if you approach it like that.
My impression is that most parents are very busy people and are just trying to keep up with the chaos caused by their kids; they will opt for the easiest solution that solves their problem.
You can get the contact details of the hiring manager on the companies website or call their front desk or ask in an E-Mail to info@xyz or support@xyz or even by asking around.
and there are various ways to prove your value that are not based on networks
Once a service gets so big, that you are practically forced to join, regulation seems like a very good idea