[utrechtsouth]

It's not mandatory. Actually it's likely illegal.

[hutzlibu]

I just read the twitter post. And from there it sounded like the app is mandatory, because everyone in school had to use them and it was the only solution - and a bad one. So parents made a better on on their own - client - but they still need the sever - so the government contractors change the server-client communication to block the competition.

So it is in effect government mandatory at the moment. And pretty stupid and yes - hopefully illegal.