Their school used to be a vocational secondary school. After years of hard work, it finally got upgraded to a high school. Only then did the school start taking academics a bit more seriously.
Night self-study was mostly set up for the boarders. Once school was over, these kids had nothing to do, wandering around or messing about. Some of them didn't have much self-control—neither reading nor doing homework—so after a while things got out of hand. That's why they established evening study sessions, as a place for everyone to do homework and study. But honestly, it was just so they'd cause less trouble.
And students like them, who lived nearby, would sometimes join evening study too. It was convenient to get together and do homework, and if they didn't understand something, they could ask each other. But it wasn't mandatory, so if you didn't go, you just told the teacher. That was enough.
