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...