I will take a few points on here in a semi-sensible way
Lots of useful posts on here. I would add a couple of bits that are going to encourage a torrent of mong buttons but anyway:
Don't believe everything your godson says as fact. Children, by their nature, find it very difficult to be impartial and will give what they think is an honest account of events. I have been involved with a few issues where the member of staff has given a factual statement and the parents believe the child's version totally. When the statements are then compared the parents complain that their child is being called a liar.
Definitely ask him what happened and how it escalated without leading questions. Just bear in mind if he was angry/upset then chances are he will put a negative slant on other people's intentions and behaviours and probably won't remember small details or polite instructions before it all kicked off.
BUT... you will know your own kids and frankly, teachers lie too when it comes to covering their arrses. If the teacher says your kid did something, demand the evidence (and not just what they write on SkyBlue/Go4Schools). Speak to the parents of the other kids in the class that you know and get their kids accounts and challenge the school. If there's CCTV footage, demand to see it. If there were written reports taken, demand to see them (use your rights under S15 of the DPA and submit a Data Subject Access Request - and when the school tells you that they won't release your child's data to you, get your kid to sign the DSAR, it's BS anyway and just a school stonewall tactic).
There is either a lot more to it than that or the school are trying to crack a behaviour problem with draconian policies. My money is on the former (see point above). I cannot think of any situation where an argument about leaving the classroom warrants a fixed term exclusion.
OR... because schools are an unregulated wasteland where HT can behave like feudal Lords, the teacher(s) involved may just be taking their frustrations out on the kids in their classroom.
Case in point: no1 daughter's form tutor is a total life inadequate. She constantly complains about his petty behaviour and the way he treats the kids in the form on one of his (her words) "bad days". He tried putting the whole class on detention because a boy from another form came into the form room before the bell, and managed to break a door handle. Everyone in the room saw this lad break the door handle but he ranted at the whole form that unless one of them owned up they would all be in trouble - problem is he and everyone else knew no-one in the form room was at fault for this. The whole thing escalated last week, the school tried backing the teacher and the kid (from the other form) who broke the handle by hanging his sports bag off it owned up, and told the Head of House that he had owned up before the form teacher went into his weapons grade rant. The whole thing was quietly "forgotten about".
If the leadership and engagement in the school were good, draconian measures shouldn't be necessary because it punishes the innocent and the guilty don't care., so never accept that as an excuse.
OFSTED have a questionnaire you can fill in about the school, and once done, you can raise a complaint with them directly as long as you have raised it with the school to begin with. They usually state that you should only raise the complaint with them if the school has not adequately dealt with the problem - but take it as read they will close ranks and do the square root of FA. Also, you need to frame the complaint in terms of the running of the school, nit just your individual case. This will then be brought up as and when the inspectors come round.
That does sound like incredibly poor leadership. Just from a legal standpoint to temporarily exclude a child without notice is probably not allowed (you'd have to double check in the Education Act for the details).
I would ask the school to clarify exactly what the punishment was (detention/ internal isolation / fixed term exclusion), what the reason for that was and how it fits in their behaviour policy (they have to have one and make it publicly available).
don't expect them to be balanced or objective, and that's assuming they don't just stonewall you.
Despite some other posters on here, I wouldn't go in smashing down the Head's door and demanding answers. Phrasing your concerns politely is much more likely to get results.
ABSOLU-FUKCING-LUTELY do go in hard and prepard to smash in the doors.
It's been said elsewhere, Headteachers are life's last remaining feudal Lords, and fukc they behave like it. The only people they answer to are the Governors who generally tend to be useless. Most are retired/semi-retired do-gooders or the only parents who are prepared to spend the time going to meetings where they never get told anything really useful anyway. They are told that they are there just to be "the critical friend", with the expectation that they are fully behind whatever the HT is doing, most of which they will never ever see anyway.
The school will have a complaints procedure. the second stage is a hearing/meeting following an "investigation" which will often be given to the most incompetent of governors to do, so expect the intellectual vigour of a housebrick where all the pertinent questions are missed or misunderstood. the panel will then consist of 2 Governors (sorry "critical FRIENDS) and an "independent" member - usually an HT from another school who is part of "the club". Your complaint will be dismissed and any evidence you do have to support your argument will be deemed
inconvenient "unusuable". Expect the following excuses to be trotted out - we can't talk about that because of confidentiality, we can't accept any arguments that appear to be legal arguments (like the Data Protect Act/GDPR doesn't apply so you CAN give me the CCTV footage that shows little Johnny DID't do what he was accused of and you CAN give me the statements from the other kids saying the teacher just picked on him and here's the specific wording of the regs that state it).
They will tell you that you can't be legally represented, but they will take legal advice before a hearing. Make sure you do the same (your house insurance will probably come with limited legal protection you can use).
You HAVE to be prepared to fight, and you have to remember the HT is on their own side. Remember - you pay your taxes and each child is per capita funded so in effect your taxes pay for your child's place, therefore they are the service provider, you are the client, and THEY are bought and fuking paid for! You have to keep that mentality or they win.