I always dread Y9 on my timetable. Y8 are not much better.
It is amazing how seemingly similar schools differ. I was teaching in one where the kids were so lazy - there was nothing that I could do to motivate them. They were uncouth, scruffy, unprepared for lessons, drifted into class over a 20 minute period. Their expectations for achievement were nil - Y11 didn't expect to get anything in their GCSEs, so they didn't think there was any point in doing any work. Other places, of similar SEC intake, were well run with pupils being reasonably respectful and able to be chivvied along.
I could never work in a comprehensive school full time, and my own kids won't be going to a comp. - I've seen what they are really like, even the "good" ones. I don't mind doing the odd day almost anywhere, but I am very choosy about where I will go back to, or where I will accept multiple days.
As for supply work, the quality of the lesson is very dependent on the work that is left by the teacher. A lot of teachers leave copying from the text book - and that is the worst kind of lesson, where the kids will chat, getting louder and louder, and very little of the work will be done. The best kind is where the kids have sevearal different tasks to do in a lesson, and you have to interact with them.
If you can get on the lists of some decent schools, supply work is a great way to get back into the profession. It's really beneficial to see how different schools operate, and what it is like in different subjects.