I remember eating disorders and self harm, alarming (in hindsight) relationships with much older men, kids who bunked off and nobody did anything, they didn't let their parents know they were school refusing, but they still did! People at my school getting drunk/high in parks from about 14. 13 in some cases. Loads of people smoked. Teenage pregnancy used to be much more common than it is now. I think these are all symptoms of deeper issues for teens, they don't just rebel for no reason at all.
I couldn't tell you what was going on with most of my school year TBH but among my own friendship group there were 4 of us and I would say in hindsight all of us were struggling with MH - I probably didn't show it much at the time but I had undiagnosed ADHD causing depression, anxiety, social isolation. My best friend W was probably autistic but outwardly, depression, restricted eating, social isolation again. Another friend C was struggling with turbulent home, weight issues, abusive ex-stepfather, very young mother who clashed with her a lot. She would talk to adult men online and met up with a few of them (!) The other in our group was L, she was self harming although nobody knew this until later. Struggling again with family stuff. Alcoholic father.
I had another friendship group through a hobby club. Group of 4 of us again. 2 struggling with weight. One with intrusive thoughts and anxiety (in hindsight - at the time just thought this was normal teenage stress), one had a really chaotic family life, another's parents divorced and she struggled with that.
I think as a child you just don't see it the same, as an adult especially in a position of responsibility you know about things. I wouldn't have known about persistent truanters or school refusal because I wouldn't have seen those pupils at school.