If this is a consequence for a severe misbehavior, then it makes sense. If it is because he is impulsive, that is part of his diagnosis and it is illegal for the school to exclude him simply because of his “disability” so you should verify the exact reason. Also, were you informed ahead of time or just the day he got to school and didn’t go? I may be from a different area but where I am, a parent with a child who is unpredictable in a new environment has the option of a known adult accompanying them and being responsible. But, I repeat, excluding him simply for his ADHD is illegal.