I agree with pps that a big part of it is that it just wasn't tracked as much decades back. I often disappeared from school and it was barely a thing back then. It was viewed as the parents and for teenagers their responsibility to take up what was offered, not the school. That's changed a lot.
Also, while there is talk on here at different times of year around all the choice parents have, many don't have many options. If your local school is either a terrible fit for your kid or just a hostile environment, it's going to cause more issues and yes, some parents are more likely to challenge than they used to.
Does not coping at school = not coping at uni = not coping in independent adult life?
Not automatically.
My DD1 did not cope well at secondary - no attendance issues, she'd get upset to the point of hysterics if the bus was late as she worried about being late, possibly getting detention, and there were weeks where every day had tears before, during, and after school. She was and still is the Lawful Good toeing the line type.
She has since worked in secondary and primary schools, no issues. It's surprised me, I was wary of her going into education with her experiences, but she's like an entirely different person.
A large part of it is, if someone's shouting at her or using abusive language at her, she's now allowed to walk away, get other staff involved or put the phone down.
When she was in secondary as a student, she often couldn't walk away and when she tried to get help from staff, she was often told she was being too sensitive, that the other kids 'didn't mean' what as an adult she is promptly told is racially abusive language that she doesn't have to and shouldn't accept. I had to intervene and threaten to make a formal complaint to get a teacher to stop making jokes about her heritage (I'm an immigrant, the teacher made jokes at her about where I'm from regularly, a place she's never been), her getting upset and asking him to stop wasn't good enough - and that's with me being a governor at the school and fairly well known to the staff, I often wondered how much farther it would have had to go if I was a 'normal' parent who wasn't well known and clearly up to date on the complaints procedure.
Oh, and it turned out her fear of detention came from a teacher who would mock students, particularly if they were the good students who'd never been in trouble before, if they got late detentions.
Unsurprisingly, when I was able to move DS2 to a different school when he was experiencing similar, a lot of things improved for him too.
Many adults don't cope well in that type of hostile environment, I don't think we should be surprised that kids don't either and can improve when in a better one. Helping kids to access those environment, and helping more schools to be those environment, is part of solution - which is hard with how much is piled onto school's plates at the moment.