What happens when Karin doesn't come back from school?

If you are referring to May 30th,then go to the woods and you'll see her tied to a tree

If she doesn't go to school she'd get detention, and that is the time when Mr. Solheim touches her :(

I also had the problem of being stuck in a time period where I hadn’t any activity’s to do to make the time run faster. Then I figured out that if you press the “clock” in the game, time will change. Hope this helps everyone that had the same problem

There’s two answers first is that klaus/Karin is in the woods she’s tied up on a tree you can only save her at night and second is that she has detention and that’s when klaus\karin gets sexually abused by her new teacher:(