But they forked a Matrix server as well to limit it to a standalone server and to limit who can access what information for the reason of privacy of the kids.

(They likely didn't fork it to make it standalone, you can do that from the config. The article doesn't mention that as a reason, either.)

They changed the login, so that you can't use anyother client.

They changed the contact possibilities, so that conact can only be made on invite only.

I read an interview with the responsible vicr-principal of the school today [1]. From my understanding the only created QR Codes that included login and password (minus last 4 digits) and the client used this info to send the request to a standard matrix server. The users only had to type in the last 4 digits manually.

Everything else can be configured via the matrix server config

[*]https://www.golem.de/news/matrix-grundschule-forkt-messenger...

Doing that is supported in synapse and just a matter of setting federation_whitelist = []

Thanks, I edited the title.

Best translator German/English in my experience is https://www.deepl.com/en/translator

Unfortunately, I don't believe that DeepL yet has a way to translate entire web pages on the fly (if it does I'm not sure how to use it) as I shared above.

Here's a corresponding Yandex translation for those who are adverse to Google, their translation service is pretty good in my experience: https://translate.yandex.com/translate?url=https%3A%2F%2Fher...