good luck being the only parent asking for that when everyone else is fine with whatsapp. it's not just the school you need to convince but every other parent too.
No one can compel you to use WhatsApp. If the school says that they need to have mandatory communication with parents and children through some kind of system, it needs to be universal.
In my kids' kita, I wasn't alone when I said that I didn't want to use WhatsApp. So we created a mailing list. Parents and teachers can still talk on whatsapp if they prefer, but any "official communication" needs to be through email.
it helps a lot if you have one or two allies who agree with you. but when you are the only one it feels very isolating, like everyone is against you. and not everyone is willing to put up a fight alone.
The first thing to change would be this attitude. Don't treat it as a fight, and don't think of people being "against you" but merely "insensitive to the matter".
Just talk with them, see how much common ground you can find. In my conversations with other parents, I found that some were interested in changing away from whatsapp, but didn't know what would be the "best" alternative. In the end they opted for Signal, still not ideal but at least it was a step in the right direction.
In my kids' kita, I wasn't alone when I said that I didn't want to use WhatsApp. So we created a mailing list. Parents and teachers can still talk on whatsapp if they prefer, but any "official communication" needs to be through email.