DS was hit in the face by a child at school

Teach your child to hit back.

The school has dealt with it. You have been informed and the school is aware of what happened. Not sure what you want to now happen.

As others have said, if anything you speak to your son about being where he should be. It sounds like they were following him off onto a field when they should have been on the playground or with the rest of their year group. If anything, explain that if worried about someone to go and tell the teacher.

I agree with your first paragraph but don't think the school can be watching all the kids all the time, so don't see it as neglectful

The problem here is this older boy probably said more than once to leave him alone and the others probably didn't.

My son has described many times when a kid has asked him to play/if he's ok and he's said no leave him alone. Two seconds later, child asks again and again and again.

How would you react if someone was repeatedly asking you to have a drink with them and you were repeatedly saying no thank you? By the 10th time you are not quite as nice and polite are you?

I'm not saying your son is at fault but you could take this opportunity to reflect with him and let him respect other people's feelings.

What Ds should have done is left the kid alone and alerted an adult.

He meant well and is obviously only age 10/11 but needing to cool down is code for someone being close to exploding and needing a time out. Being crowded by all the kids would have set off the child. I’m not saying that your Ds was unkind and it was his fault he was hurt but if someone asks to be left alone then they should.

The older boy shouldn’t have hit. The younger ones should have left him alone.

Wait and see what school say, @TotallyAddictedToCoffee blaming of your son only works if the 7 yo has knowledge that that's what the year 6 was doing, and they shouldn't, should they, given the usual MN response of 'it's none of your business other child's needs'?

The school are dealing with the incident, what more could you do?

I think you need to get to the bottom of whether they genuinely went to see if this child was ok, or to gawp/wind him up further.

Does he know the child well considering he's 4 school years above him?

I don't think the blame lies with the boy, who had the wit to go off alone to cool down and try and settle his emotions.
The school should have been monitoring this situation and it's their neglect that your child got hit.

Sounds like you need to teach your DS to respect other peoples space and not follow someone who is "cooling down" - I get that he meant well, but he's just exacerbated the situation