I am mum to a severely autistic boy who spent 3 years in mainstream. I have a slightly different point.
I actually think the law is quite a sensible one in the case mentioned by the OP.
Every child in the UK has a right to a place in mainstream school, whatever their needs or disability ( I am quite proud of our country for that: it is certainly NOT the case in many many other countries, far from it).
However, the education of a child with SEN must not be "disruptive to the efficient education of others"
If it is proving disruptive, as in the OP, the school must take "all reasonable steps" to adjust to the SEN child's needs - this could mean giving him/her a quiet room/desk in times of stress, or a differentiated curriculum, or more training for the TA (often the best starting point)
It is the same principle as - don't just tell the kid in a wheelchair they can't attend the school because of all the stairs, put in a lift to enable them to access the mainstream education which is tbeir right.
However I don't think it is fair for the situation in the OP to go on forever, to the detriment of the rest of the class
That is to place the rights of one child too far above the other 29
When my boy was noisy or disruptive in class, I fully expected him to be taken out so that everyone else could hear the lesson
I also worked (and work) very very hard every hour of every day on his behaviour, such that he can access as much of the world as possible. But he has to learn, eg, to be quiet in the cinema, otherwise it is not fair on everyone else. If he isn't, I take him out. Then he learns to keep quiet, as he wants to watch.
But I think it is good that these issues are debated and I think there have been some very supportive posts on here
.