"She told 2 boys to stop talking and they just walked out of the lesson!Got brought back by the head, and they just did the same thing again."
Sadly, Smowball, this is not uncommon in many non-selective schools. In most classes there will be some students who openly ignore instructions to put their phone away, stop eating crisps, stop throwing other students' books on the floor etc and when challenged too many times just walk out, perhaps kicking a chair over as they leave.
It is very difficult problem to tackle. Exclusion from school has to be a last resort. We had to move one particularly disruptive student out to a special outside facility and I heard that it cost the school something like a thousand pounds a week from its budget. Detentions and phone calls home generally have no effect for these students. The hope is that these students will leave a and go elsewhere. However schools often find they have to take in disruptive students on a managed transfer from another school.
One way round it is for strong senior teachers/heads to constantly patrol the school, looking into classrooms, and to have a zero tolerance approach to misbehavaiour, but sadly most heads do not have the time or desire to do this.
If it's any consolation, if your son is in top sets the disruption will be fairly minor compared with what goes on elsewhere