Completely right. It comes to funding. There should be smaller classes and more staff, that way, being in the bottom set wouldn't necessarily mean being in a class full of naughties to.
In an ideal world, there should be zero tolerance on behaviour. The disruptive element should be removed from all classes, regardless of what classes they happen to be in.
The sad thing is, for decades, government, local authorities, school governing bodies and heads have buried their heads in the sand regarding behaviour. They refuse to accept that behaviour in our schools is far from good. Until they accept that there is a problem, the real issues won't be tackled.
While we wait for that time, while not ideal, mixed ability is really the best solution for the majority.