One of the difficulties is that classes don't have just one kid with SEN in them anymore - you could have three with ADHD, one with emotional difficulties, two with ASD and a couple with anxiety. And two little shits.
Add into that a low level of literacy, dyslexia and, all too often, shit home lives and as a teacher tries to get the class settled, something happens and it kicks off, especially if there is always one or two who can be relied upon to not just disrupt other kids' learning, but are also quite unpleasant to the other children on the whole.
What else can a school do with somebody who, for example, will argue that everybody's picking on them, but apparently forgets that they repeatedly call another kid a retard because they find writing difficult, sings 'Dead Mum, dead mum, dead mum' at the kid whose mother died of cancer last year and gets in the face of another who has ASD and screams two inches from their face until they are practically catatonic with terror?
It's not fair on the other kids to have to put up with that.
It's not solitary confinement, it's a room which is peaceful and low on distraction/stimuli/sensory input - and many students have been known to deliberately play up in classes, just so they can get themselves somewhere where they can work uninterrupted (resources for each subject are provided).