I think the fundamental problem here is the lack of funding of SEN provision in schools. Students aren't supported adequately and this can then lead to a raft of problems for all the students in the class.
I've had to deal with funding issues in school - the horrendous problems when statements became EHCP s (I know of a LA that said they had a backlog of several years!!!!) Our school had to fund the first £6000 and when every student only received £4000 anyway and we had a deficit budget it was virtually impossible, many students didn't receive enough funding to match their needs (so maybe funded for support only at certain times when in reality they needed 1:1 at all times). The truth is that LAs do all they can to pay out the least amount of money and then everyone - students with SEN, other pupils and staff - just have to deal with it.
I was working in the NHS at the time when psychiatric hospitals were closed in favour of "care in the community". At the time it was argued that long term patients became institutionalised, locked away from society and that they would live happier and more fulfilled lives in smaller community homes, whilst being supported. Sounds exactly right in theory. In practice it was under funded and many patients slipped through the net and ended up struggling on their own, homeless or in prison.
It seems to me that SEN provision has gone the same way. It is sold as being good for everyone - integration rather than segregation - which is how it should be. Except in practice it seems to be code for "do it on the cheap" and everyone suffers.
It must be terrible for the OPs son to be so distressed at school and it must also be terrible for the girl in question, being attacked every day at school. If the school is anything like the one I knew, the staff and SLT will be only too aware of how dreadful the situation is for everyone but there is simply no money in schools to change this.
A PP asked why the NT students needs have a higher priority - IME it isn't that simple. If you have 29 NT students and 1 ND student what choice do you have? As a school you have to educate the majority. It's terrible, it isn't right. But these decisions are being faced in all public services - do you meet the needs of the few or the majority, when faced with a shrinking pot of money?
The girl's mum should not have approached your son. Someone needs to get to the bottom of what is happening so I think you have to speak to the school and try and come up with a plan to make sure your son's needs are better met.