While I agree that you should have asked to sit in that ADA chair or taken it yourself I also remember being that age and would have done anything to avoid calling attention to myself. After reading your article we need to realize it is important to know that others go through what we thought occurred only to us.
It is also important for all of us to call out behaviour like this where we see it. Someone should have spoken to the teacher privately and arranged this. I have a rather overweight friend and when she is over I always have a sturdy chair for her without making a big deal out of it. If we want a civilized society we must stand up and speak up for those who are either not able to or are embarassed to.