I think some of what you described is a bit over the top. I think that asking him to rewrite his name wasn’t out of line, but him getting in trouble for it was out of line. Even if he was being silly/attention seeking that may very well be because he doesn’t have the language skills or emotional skills to explain that his bad handwriting was due to ADD or motor issues.
I also think the longer he’s out of class, the further behind he’ll get and ultimately that isn’t worth whatever lesson he’s supposed be learning. Also, why was he drinking juice at recces? Was he simply thirsty? was there a reason he had to drink then? they need to work on figuring out why he broke the rule, oftentimes it’s more complicated than just him being insubordinate.
I think oftentimes schools in trying to punish kids with ADD forget to focus on finding a punishment that “fits the crime” and that actually teaches your son what he needs to learn.