I did it!
I have to delete the timetable or put it off. Now I can click on the board or the schooltable to manage the bills, and they doen't vanish anymore ^^ Thanks for reading ^^"

im having this issue too!! lmk if u find a fix

Thanks ^^

It is a bit more complicated now than it maybe should be ^^ since the new schedule-system/timetable and the old way of doing things are both there now, to not break support with older saves etc. or for people who prefer it.

If you use the timetable, the bills are all managed on their own (I may need to make that better visible with a future update ^^)

So you can either use the timetable on the blackboard and also use the same menu on the school desk itself (they have it aswell) to set up each student with their own desk (so they only go there and no one else, that way there is less chaos ^^)
(I have added a new screenshot at the end to maybe explain it a bit better)

Or you can set the bills for both yourself like you did now (but you are limited in how many lessons you can have and at which times etc.)

My guess is if you really want it based on the skill, you likely will need to do it as you said. (I will see if I can add a general option to the schedule to only teach up until a certain skill level, sounds like a good option)

Edit: I also do understand German ^^