This is the thing col, they do assess you to "ability stream" as you put it, but only the first year you start secondary school. They then decide what "set" your in and that's that's until your GCSE year when they have a bit of a reshuffle. This is also assuming that these disruptive kids are all idiots, which unfortunatly isn't the case! Some of them are very smart but due to a lack of discipline and an emmense boredom throughout their school lives don't have any respect or self control! There were 1 maybe 2 teachers in the whole of my school that could control a class, both of them from an older generation of teachers where discipline wasn't frowned upon and somthing to avoid at all costs. There were also the only teachers who engaged with the class and made the class content interesting and even somtimes enjoyable. When working with these teachers some of the kids who were disruptive and belligerent at other times came through as some of the smartest kids in the class!
Now I'm not saying its all the teachers fault, although you do have to wonder if one teacher can do it, why can the rest?!