> 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.
Getting your own kids to switch messenging client is fairly trivial. Then they can talk to you (and to each other).
The network effect means you then have to persuade other kids, and parents, and sports coaches, and music teachers and ... to switch.
(Source: have three kids and we have exactly this issue)