I agree that maybe your son could have handled it different and thus should have been shown that issue in a different way. Like maybe the Teacher should have went up to him and said " Honey that isn't necessary to say, no one is going to bother your jacket" or something to that effect.
I'm sure that was what the teacher was trying to get across but did it in a poor way.
I may have done that to my own child in my house but certainly not with a crowd of kids.
I don't think it's the worst thing but maybe a heads up to the teacher that there are other ways to get the same point across may be in order.
I know that now days even the Teachers with the best intentions seem to get nailed on all kinds of things which leads them to not wanting to instruct children on even basic manners in life.... So IMO a gentle guiding on this issue may be warranted.
Let us know what happens
God Bless