Isolation should be used for two reasons:
To allow a disturbed class some peace from persistent disrupters.
To spend time on more 121 basis with offending students. They should have access to appropriate work, and their time needs to be spent productively.
Often these children are some of those with the biggest problems and time can be spent talking and extrapolating information from them can can help in the classroom.
However, they also need to reflect on their behaviour and be willing and able to be reintegrated.
A week in a refocus room with good support can set a minor offender straight again.
However, we do now have more seriously damaged (from neglect, poor parenting and various other reasons) kids in schools and sometimes behaviour cannot be controlled.
There are a great many students who would benefit from being in alternative provision at secondary as mainstream us just too much for them and unsurprisingly, they act out from fear, anxiety and misunderstanding. This doesn't excuse the behaviour, as their classmates deserve a decent, undisturbed education but what can schools do with children who cannot fit into the social model expected of them?
It's a no win situation. Isolation = failing those kids. Having them in class = failing the other students who's education is affected. Every way the schools cannot win.
Schools have no support. They're doing their best with spectacularly limited resources and funding. TAs and SEN provision is cut to the bone already. There's just no bodies or money.