You need talk with black hair girl about her sister.. with low hp
Nothing happens after that for me too just the black hair girl saying i need to prove her sisters innocence but i don't see any prompt for that.
you should do all brown hair girl's request and talk with yellow hair girl.
did that again, no change, same result
you can talk with black hair girl as "Your sister has been sent to the infirmary " with low hp. if you clear the black and red hair girls' quest, you can talk with yellow hair girl about teacher's wallet.
It's still not working what even is supposed to happen afterwards?