A very interesting and thought provoking read. If I understand you correctly, you are agreeing that the chair design is inherently coercive, and so is school, but that this is ok because coercion isn't inherently bad. 

Of course this is true. We coerce people all the time. Road rules, workplace safety rules, taxes etc. We have coercive rules on one side and on the flip side we have rights to protect us from unjust coercion. 

Students have rights to protect them from unjust coercion. One of those rights is the right to self-determination and choice in education. A system that removes student agency in the name of learning isn't a just use of coercion. It violates their rights. Making an ADHD or ASD student sit still isn't a just use of coercion. They have the right to reasonable adjustments.