Speaking as someone who was a 1:1 for SEN in mainstream and now work in a specialist provision. Typically if a child with SEN is accepted into mainstream then it's accepted they can access the work so would be too high in a specialist provision. However this doesn't take into account behaviours. Going off my own experience with supporting SEN in mainstream, mainstream are not given the funds labelled in their EHCP to meet their needs. Those funds are distributed across the school to plug the gaps of funding for other areas. It's not right and it's not ok, but given the government has slashed funding so much it is now almost impossible for SEN kids to access mainstream as their simply isn't the funding or training offered to deal with it.
On the other hand, specialist provisions deal with high behaviours and low awareness that make being assaulted three times a week look mild. Just today I've been bitten twice, had my hand bleed due to scratches and at least half a dozen new bruises appeared on my legs and arms. Yesterday a child was thrown off the top of a climbing frame due to another child being in crisis. This isn't to say what is happening to your child is ok OP, just to highlight the differences in a SEN provision Vs mainstream.
SEN take the most extreme and can't take those who can but struggle to access mainstream due to the sensory ect factors which leads to behaviours because they are full up with n children who most days are behaviour management. This then leads these children to mainstream, without funding or strategies, taking in the less extreme pupils as they are too high for specialist provision but too low for mainstream. It's why the goverwnt need to reintroduce arcs across the board for children who are able to access the mainstream curriculum but need support to do so. In an ideal world it would work but it doesn't, because children can't be seperated into two extreme categories.
In your case OP I would definatly take PP advice and keep everything to email and take things higher if the school doesn't recognise their duty of care to all pupils instead of the ones with SEN. It's ok to be mindful of SEN but at some point the other children shouldn't suffer. Children with SEN are capable of knowing what's wrong, it just takes a lot more harder work. But if you don't you are left with an adult who is locked up due to biting people who say no to them as an example.
Your child shouldn't be made to feel unsafe, and while it's ok to show compassion to the other child it ultimately not your problem. Fight your child's corner and make the school acknowledge it. It would benefit the child with SEN and your own long term if you do. So go to the governor's and LA and if that doesn't work ofstead and the media. If they have taken this child they have accepted a duty of care to help him, and the children he comes into contact with. Don't let them fob you off.
Best of luck xxx