You are being fobbed off
It isn't a school issue because the children involved have statements of special educational need, which means that the LA have responsibility ultimately for their specific individual education, not the school, although the school act in loco parentis.
I would write:
Dear
I am writing to inform you of an incident at my DS's school on which has had a profound effect on my DS's well-being.
As you may be aware, last year my DS was signed off from school by our family GP as he was displaying suicidal ideations. He had been assaulted by on several occasions, and as a consequence wanted to die.
During a meeting to form DS's current IEP, we were assured that the child who had been assaulting our son was being given 1:1 supervision during break times. I expressed concern that my DS was being kept in during break times. It was agreed that as the other boy now has 1:1 during break times, DS should try 5 minutes play time outside.
On DS went out for the agreed 5 minutes. However, the boy did not have 1:1 as promised, and was able to make a fast run towards DS, scaring him. As he was leaving the school with his 1:1 TA, DS said "I wanna jump in front of the bus." The TA was concerned enough that she had to keep a very tight grip of my DS. He had also wet himself during his short time at school, despite being fully toilet trained.
I am writing to ask that you take action to remedy the situation my DS finds himself in. I understand that as both my DS and the other boy have Statements of Special Educational Needs, it is your responsibility as Chief SEN Officer to make the provision as specified in the Statement.
I am not satisfied that is a safe environment for my DS, as he now has suicidal ideations as a direct consequence of his attendance at the school.
I would appreciate your full attention to this matter, and await your response within 7 days.
It is simply unacceptable that a 5 year old can be driven to suicidal intent as a result of a school's inadequate safeguarding. The previous year's events have made it clear that my DS is at risk of harm, and this needs to be taken more seriously.
Yours sincerely
want2sleep
Needs major rehashing, but something like that.