I'm going to blow up at a classmate because of sensory issues
What did that person do to make you angry?
He put his backpack on a chair that has been occupied by my bag for 30 minutes already.
You should just tell them you has that spot first. Blowing up at then is mean and will only cause more problems.
I don't care what others think, I only matter.
Don't you think it would be friendlier to simply ask him instead of blowing up at him? He could be a really nice kid and would actually make a great friend if you be nicer! He might not realize that his backpack is on top of yours.