Lots of factors over the past 50 years have contributed to the perfect storm we see today:-
1- School sizes - we have fewer and bigger secondary schools making it harder for those who struggle with noise, crowds, etc. It also makes it more likely to need transport to schools further away, making kids suffer the often awful conditions of noise, disruption and bullying on school buses.
2- Comps - we used to have different types of school for different (broadly) types of children, i.e. grammar for academic and technical/sec mod for non academic. That meant kids could, generally, go to a school more suited to them. With comps, you're fitting square pegs in round holes - no academic kids are forced to do academic stuff they're not interested in and incapable of, at a young age, and only allowed to choose subjects they are interested in for GCSE years, by which time, many have lost interest and become disenfranchised.
3-Subjects havn't changed with the times. We're still teaching things to memorise, facts to learn, which is bad enough for those academically able, but painful for those who aren't. In Maths, there's no point trying to teach pythagoras or simultaneous equations to kids who havn't mastered the basics. Same with English trying to teach Shakespeare to kids who can barely read or write. Kids who struggle to learn will soon become bored and disruptive.
4-Lack of discipline - teachers seem frightened to discipline kids, so some classrooms become warzones with disruption, noise, etc., which are obviously harder for "quiet" children to cope with.
5-Classroom layout. Quiet kids are generally happier working on their own, on a single desk, all facing the same way, and not in the clusters/groups of modern classroom design.
Quite simply, it's mostly down to bad behaviour which affects "quiet" children the most, who need a quiet and calm environment, whether in the classroom or the school bus. A few decades ago, poor behaviour wasn't tolerated and was nipped in the bud, maybe helped by smaller schools, so the noise/disruption wasn't so bad.