@MissOlivier , firstly I am so sorry for the mess this thread has become. As an autism mum myself, I understand your concerns. My ASD son is 9, and has always stimmed by twitching and mingling his fingers in front of his face. This has developed over the years to coming behind his siblings and shaking their shoulders so their heads move, or being right in their faces with his fingers then screaming. Thankfully, it hasn't happened to anyone outside of the family, to the best of my knowledge, but we ALL hate it. We have tried over the years to reason with him, punish him, shout and scream at him, talk to him, do it back to him. I am genuinely at my wits end and whilst we aren't at risk of a serious injury, he is getting bigger and stronger every year and I wouldn't be surprised if we found ourselves in a situation like yours in a few years time.
The thing is, DS is high functioning so firstly we haven't shared his diagnosis and secondly those who do know are rather dismissive. Thé agression/dépression associated with ASD comes out at home, after he has spent the whole day at school 'trying to appear normal and fit in'. (He has been on daily therapy since 3.5).
Now, if I was in your shoes... I would go and talk to the other parent. I would talk things through with her, and agree on a plan. I would encourage her to talk to my son, to let him know in an appropriate manner, what he was doing to her daughter, how he was making her feel. Hopefully, she would be receptive and hopefully it would start to resolve. Sadly, it wouldn't be an overnight thing, as he would have to learn to control his impulses, or find an alternative release. Equally, she should be encouraged to stay away from him, and if she cannot, to control her loudness. 'Inside voices' is not a difficult concept, and is generally considered considerate behaviour towards all.
'Keep your hands to yourself' is a motto EVERYONE (NT/ASD/CHILD/ADULT) should follow. And we can only lead by example.
Please keep flagging it up with the school, for both the children's sake. Teachers are not magicians with endless patience, time and resources, but they can only manage the situation with the information they are given. Communication is ALWAYS key.
Good luck. X