This is a hysterical, melodramatic response.
something horrific has happened to another child
OP hasn't said what's happened to the other child apart from the pile on. She's said that the child has been 'fine' with her son since the incident, and that they were friends before. Something has happened, but 'horrific' is a ridiculous stretch.
and not only does your son know exactly what happened but is refusing to tell you and is trying to get away with..
You have no idea what her son 'knows'. He's already named the boy he says did it. He could have seen everything or nothing. He could be telling the truth about the boy who did it, or he could be throwing him under the bus to save his own skin.
Point being, if you assume the worst and treat him like he's guilty, he's hardly going to open up about it.
I would not be telling my son I’d stand by him regardless, he should be in his room.. No phone, no iPad, no anything.
Why would you not stand by your child in this situation? It's a bit of rough and tumble at school that got out of hand, he hasn't killed anyone.
From the information OP has given this is seemingly out of the ordinary for her son. All that overreacting, shutting him away and not standing by him the first time he slips up is going to do is ensure that he never feels comfortable in coming to her in the future.
My parents always made it clear that they would have my back through anything, but in return they demanded absolute honesty. This doesn't mean that they covered for me, or that I wasn't punished.
He needs to understand that he needs to help fix whatever has happened to another child because it’s clearly something horrific before the police turn up at your door.
He's already named the boy he says did it, and apparently reconciled with the boy who got hurt. Assuming he's telling the truth, what else do you want him to 'fix'? OP has already given him 'the talk' and will punish him for being involved in the pile on.
It is not 'clearly something horrific' - if the boy had been hurt badly enough to warrant police involvement, this would have already happened.