As a foster carer who has cared for many young people with serious behavioural issues I have the following thoughts. I understand you are upset/annoyed and would feel justified in going into the school and making a complaint. You are of course entitled to do that and I do not think you would be unreasonable for doing so. I hope you and your son are OK.
As a foster carer I cannot tell you how many courses we have been on about not touching a child, incase anything could be misinterpreted. However, in reality this is a rule we break. Not just for actions with a negative conotation, but in physical hugs and hand holdingtoo.
One child I cared for went to a school where due to a parent's earlier complaint about a similiar issue (a complaint that may well have been fully justified too) lead to no member of staff touching a child under any circumstances. Unfortunately the child I cared for soon learned this and used it to his own advantage. We were not expecting the child to be physically restrained over a minor issue, but a gentle pat on the shoulder and being lead quietly away, would from my knowledge of the child have worked wonders. But the staff were not able to put their hands on him.
On another occasion another child ran out of school and across a busy road, where again the staff were not allowed to touch him. My husband who happened to be there caught him and lead him back to the school. Shock/horror my husband broke every rule in the book, but also saved a potential accident. The child's parents btw later thanked him.
So I guess what my ramblings are saying is that a minor justified complaint like yours, can sadly sometimes result in a zero tolerance policy, that can have an effect on others. Reading that back I realise that may sound like you should not report, but actually I do not mean that. If you genuinely feel justified then you must, but if on honest reflection her action was an acceptable one then maybe you shouldn't.
Is there a middle response of a quiet word?