Chardy Unfortunately, I don't think Gagajo is wrong. Disruptive pupils aren't confined to SN or "bottom" sets. Some subjects in some schools only have mixed ability groups anyway. It is not uncommon, for example, for MFL to be taught in mixed ability groups in KS4 because the timetable won't work any other way. Some disruptive pupils are very bright and the dilemma is whether to put them in a "top" set, where hopefully the majority will get on with their work or in a "bottom" set, which might be somewhat smaller.
I'm not going to quantify the number of hours lost through disruption because I don't know. It's anecdotal, but it's based on my own experience in two schools, where I worked for a short time (fortunately I was able to leave quite quickly), the experience of one of my own children (again I was able to find her another school) and what my tutees have told me. All of this took place in an area which wouldn't be considered deprived. I did spend most of my teaching career in schools where discipline was acceptable, but there most certainly are schools (and not just a tiny number), where the poor behaviour of a few pupils disrupts the education of many.