I would be complaining to the police about the 2 men that chased and held your ds - not quite kidnapping but would have thought that there was some legal technical term for detaining minors; if they weren't the ones that had done the crime then they could hardly be claiming that it was a common law arrest...
You don't say where they were being held, but even if it was outside I would be furious with the men that did this. Can't begin to imagine how scary it must have been for your ds to have been chased and held.
What did the police say about the way the men behaved - can you get them to caution the men?
Also - I would be furious with the teacher that took the boys names. If he stuck his nose in for long enough to take names then he was getting involved; if there were boys there who were being held for doing nothing wrong I would be outraged that he didn't help your ds & the other innocent one.
We try so hard to give our children messages about being responsible and keeping away from strangers. And then the message gets all muddled when things like this happen.
It also sounds like you need to speak to the head of house to make a complaint to him and ask exactly how a young lad can bring a school into disrepute by being chased by two scary men who they had no idea what they were going to do to them - surely getting away was the sensible thing. What if the men had been annoyed and rather than wait for the police thought they would dish out a little of their own justice and beat them up for the broken window, regardless of their innocence? It's easy for him to get on his high horse with the advantage of hindsight but he wasn't there, the boys were and they were scared and had adrenalin flowing, just a moment to make a 'fight or flight' decision - they made the right one.
The worrying thing is that next time (hopefully there won't be one) the wrong message has been sent out, not just to your son but the school that you should stay in potentially dangerous situations - just to avoid saturday detention...
When you spoke to him on the phone was it during or after he'd been held?
If it was me I must admit I would have been straight on the phone to the police to say that some stranger had held my son captive. I would also have wanted to speak to the men.
Sorry, lots of random thoughts, reading this made me very angry on behalf of your son and you!