I had a similar incident happen to my child in year 8 - it was not the first incident but it was the worst. I sent the following e-mail/letter to the school and governors, it totally stopped the problem in it's tracks.
In addition my son was (eventually) diagnosed with a SEN in y9
Take/change the wording but basically this talks in school/child protection language and if ignored would mean that there would be serious questions to answer. Don't forget that you can complain to OFSTED, at any time.
In December I made a complaint that my child, x - in Tutor Group x (Miss x) was being harassed on the school premises by a child called x. This child was intimidating my son and wrestling him to the ground in the playground, this is common assault.
My husband, x and I came into school to discuss this and another issue and we were assured that the matter would be dealt with.
x now tells me that this child is still harassing him, and whilst my son endeavours to stay away from him at breaks and lunchtimes, as they are in the same classes, inevitably they have some contact. My son says that x continues to punch him and poke him when there are no staff present.
My son suffers from stress migraines that have seriously affected his attendance this year, no doubt in response to the sustained abuse he is receiving whilst on your premises. When I ask him what staff attitudes are to x he tells me that the staff expect this child to misbehave and that he is rarely challenged. I understand that my son is not the only child that is in fear of him.
You have a duty of care to all your pupils to ensure that they feel safe and supported in School. The fact is that my son does not feel safe in your care and this is having a direct impact on his ability to learn.
When I came in before Christmas I said that I expected that the staff would ensure that x did not touch my child again. I was assured that you would put in place measures/sanctions to ensure that that would happen. Clearly those measures have failed.
I will be taking further action and reporting this harassment to the police if this matter is not dealt with immediately.