Stundenplan is a third-party mobile client for the Untis timetable.
Stundenplan correctly reads the timetable from the Untis API and displays it to you - customizable.
I documented the whole process in my high school research paper, which you can find on my website. Note that this document is in German. Basically, I reverse-engineered the Untis API with HTTP-Toolkit and wrote a Flutter app to display the data.
If your school uses Untis and doesn't provide individual credentials, I'd be glad to support you to get the app running for you school as well! It should, in theory, already work, but you never know. If you need credentials to test the app yourself, please contact me.
Pull requests and issues are always welcome.