Oh Harman this is awful.
my DS is the same age. We moved his school because he was being bullied by teachers believe it or not.
However, that's by the by.
When my DS has had problems with two particular boys, we went to speak to his teacher exactly the same as you have and it has now been sorted.
The teacher's and the head's attitude stinks. They are trying to turn it around on you and your ex. I would demand to see the school governers, if not go to the education service and complain.
Your DS is in the care of the school when they are there, so they are resposible for their well being. If it was the other way around, they would soon be on your back about it so fight as hard as you can!
Good luck! You have my full support!