I think the idea is that if the class was more tailored to the pupils then the behaviour wouldn't be as bad because they'd engage more.
I don't know whether you can read music, but imagine being in a music lesson where it's expected that you can and that you should be able to play an instrument. You won't explicitly be taught to read music, but you can probably, after a bit, recognise middle C and read the treble clef by counting up. You have to take music every year, and over the years the rhythms get more complicated, people are starting to play Nocturnes and concerto movements, and you can still only really manage Three Blind Mice on the recorder because you learned it by ear.
Wouldn't you be fed up and feel demotivated when you went into a class that wants to talk about analysing chord progression in Beethoven? You'll either sit quietly with no idea of what's going on, or, if you're a different type of person say there's no point in being there and maybe be disruptive?
There are people who feel like that in every lesson. And the answer isn't to make them all take A-level music.
It's to realise early on that they need a different type of class — one that teaches reading music, assuming little knowledge. Introduce the instruments in an orchestra, listen to famous pieces from films and video games and listen out for the flutes or the brass. Talk about why music is important in everyday life. Group teaching on the ukelele, singing, and clapping games to improve knowledge of rhythms. They might never get to grade 8, or analyse a chord progression but it's fine. Some might only do it for a couple of years, also ok. They can appreciate music, and if they want to join a local community choir or band they'd be able to.
And having a whole class with that as the plan means people don't feel embarrassed that they're behind or like they'll never catch up with the person who has just done their grade 7 violin at 13. And learning music properly, even if they don't cover as much, means they can go to a concert without feeling like they don't fit in or shouldn't be there.
(Ok, it's stretching the analogy quite a lot, but hopefully it makes some sense. Another example would be
swimming lessons that keep moving on to different strokes when you still haven't managed to take a foot off the bottom.)