In C4 we have Dobashi asking what'd happen if an adult found out about all the lewd stuff they've been doing and then in C19/20 when Yoshida and Dobashi have sex outside we see the figure of a person watching them in the dark (pages 074 and 081 in volume 02). Next in C30 it's revealed that the teacher was the figure watching them and says that unless Dobashi goes to his house he'll tell her parents. While they're doing stuff he mentions how he figured out what the three were doing in classroom during breaks due to the smell. Then in C33 when all three are together again having sex Dobashi mentions how the teacher intends to have the two girls become his whores, naturally they don't really want to be his whores and they tell the authorities what he did with Dobashi. Subsequently the branch school is closed down and they go to a normal school from then on.