Excluding DS meant two things. ONE rejection from them as he insisted they were mad at him and he really wasnt wanted. TWO He would then use it to stay at home if shouting was all it took.
He did not understand at the time (very immature 5yr old) what he had done wrong. Which of course he wouldnt seeing as running hiding and yelling was his only method of escape from stress.
What is it they are making him do that is causing him to be this anxious? Yes demands are placed on him but far to many by the sounds of it.
Could they not just let him stay with his friends in the class and leave it at that just to allow him a period of settling. It seems the removal is a form of rejection to him by the way you have written it.
I could be wrong but would there be away of getting a rough draft of his timetable? Of what they EXPECT of him in a school day?
Worth trying saying something like you wish his excluded days to follow the same routine as a typical school day right down to putting his uniform on in the morning as you feel such a gap away from school would only make it harder to settle when he gets back? Anything really just to get a gist of the timetable :)
Perhaps it will then be possible to discuss each day with your DS in a casual way as in "what do you have to do at school now DS? you have to help me as I am learning what your day should be like" to prise his normal school routine out of him.
This could well be your gateway to seeing just how much stress he is under and of course proof that they are not meeting his needs and the fact that the school is, in fact, escalating his distress resulting in aggression 