OK, time for the Aussie perspective.
First - it does sound like this other boy is a target, and has learned to respond inappropriately to ongoing bullying. That needs to be addressed.
Second - in the game, other boys began to tease the boy with the speech impediment. difficult child 2 knows this boy has a reputation for attacking by choking. I find it extremely unlikely that difficult child 2 would know this and not the other boys, therefore I believe they chose to bully, deliberately, to provoke a reaction. They got it.
Third - I suspect difficult child 2 is less socially adept than the other kids and opted in to the teasing due to imitation. He also was perhaps less capable of seeing how close to exploding the other boy was. I used to see this kind of problem with difficult child 3 and the bullying he copped at school. It sounds like difficult child 2 joined in with a "me too, ooo ooo ooo, I wanna be like you ooo ooo" and as a result became the main target. In a way, it's possible the other boys were happy with a result that had two weird kids attacking one another. It's like kids who each bring along a dog to the park, then those kids work their dogs up, each of them, then throw them together to have a fight. re the dogs to blame? Or the ones who have set them up to fight? Even if this specific outcome was not the aim, there was certainly undefined troublemaking as the aim. It needs to be punished.
Fourth - the school's response. Or lack of response. Your son rang you, not the school. But your son was the one who was unconscious. This means there was time when he was unable to do anything for himself. Who was in control at that time? Why did it have to be your son to call you? Why was no ambulance called? I'll tell you why - because this school's first response appears to be, "oh dear, this makes us look bad. Let's go into damage control." That is a HUGE concern, especially given the previous points, especially the other boy's use of choking as a response to his frustration.
Fifth - the boy has choked others before but never to the point of unconsciousness. This tells me that the problem could be escalating. He is also getting older, bigger and stronger. It cannot be allowed to continue.
So what do you do? You could call the cops, but I think pressing charges would be complicated and may not achieve anything constructive. The best outcome would be to see a change in school response, and something being done to curb this kind of behaviour from ALL the boys. You do what it takes to get this outcome, you do what it takes to avoid anything less helpful.
Here, I would call the cops and talk to them informally. Ask their advice. Ask what programs they have that could be used to help. I would not be pushing, at this stage, to have charges pressed. But I WOULD be investigating the rules concerning what the school's response should be.
Again, I would also be calling the education authorities to determine this - the school's expected response in this situation. Again, I have the option of doing this informally or anonymously, especially if I don't ant to drop the school in it if they technically did nothing wrong.
I would also be keeping my son home until my concerns for his immediate safety at school have been resolved. I would put my questions in writing, but hand them over personally. I would add the phrase, "I require a response in writing." This forces the school to take some kind of a stand, to make a decision (hey, any decision, any time now, I'm waiting) which sounds to be to be a challenge for them.
If the school is unable to appropriately respond to these questions (which should include, "Why was an ambulance not called?" even if you suspect the answer is because they didn't want this to get out) then I would begin to take the matter up over their heads via the education system.
If this other boy felt so frustrated that he had to throttle someone, and regularly uses this as a coping strategy, then the school has a history of not handling this well and not helping this boy. Your son is another similar boy in that school and his welfare is obviously compromised (demonstrated in being advised to avoid the bathrooms between classes, as an alternative to, heaven forbid, actually trying to maker them safer).
This problem is much bigger than it seems, and runs much deeper and more long-term. You have an opportunity to be a force for change. If at any time the school gets difficult or touchy, point out that you have chosen to not press charges at this stage, because you want to see a long-term positive outcome from this incident. But the charges you are considering are not ones of assault against this other child, but of failure to protect (or whatever else you can throw) at this school.
Too many parents get to this point and, when it begins to get too difficult of they feel it is beginning to cause repercussions on their child, they back off and drop the issue. That's why these problems perpetuate.
I know I offend people sometimes, because I stand my ground and don't walk away, until I have done all I can. But I have found that anyone who gets offended by my approach, thoroughly deserves to be offended. I don't fuss about staying in the good books of someone whose opinions I have no cause to value. And if my child is getting disadvantaged because I have chosen to take a stand, I will fight on that front too. In Australia we have "whistleblowing" legislation that protects from ANY repercussions, those who alert authorities to a problem. We're actually dealing with this with easy child 2/difficult child 2 right now - she has a problem with her college, she tells the disabilities support person, then next lesson her teachers are upping the ante. As a result, she has been increasingly reluctant to ask for what she has been told she is entitled to. This is wrong, but a common tactic from those who resist the need to do their job properly. Attack the messenger.
So to take action requires strength and determination, not only for yourself but for your child. Think before you do or say what you feel you need to - how will you handle this issue too if/when it happens? Can you follow through to the last part of the task? Because to start things, then drop them halfway and incomplete, is actually worse than doing nothing.
I have seen things through to their conclusion enough times to know that eventually, the opposition backs down. Sometimes it slumps into sullen, inactive silence, but I HAVE, almost every time, seen the repercussions stop and sometimes reverse. But sometimes before it gets that far, it can get nasty and a personal attack (on you and your child) can be part of the "I do not want to deal with what you are raising" response.
My children have seen me fight, and win. They have also seen what happens in the process. They know that I don't fight every battle. Only the ones I feel are winnable and not too muddy. They have also seen the seamy side of the response and have learned that first, I believe they have been worth the effort to fight for; and second, winning and causing positive change is possible and worthwhile.
Marg