Thanks for the feedback. Just to clarify, the detention will involve only one night late after school. I sort of can see that the head had to punish the crime whatever the circumstances and, reflecting on things overnight, I think the real issue for me is that the school have done nothing about what I told them back in Sept/ Oct that DS1 was finding it difficult to find friends this year - and, later, was being pulled into the control of the X - the 'bully' boy.
I hasten to add that I don't want to condemn, globally, this other child, who i know has problems. I just don't want his problems to adversely affect DS1 - who has difficulties of his own.
DS1 (and his twin) joined this school in Yr 3. The largest cohort in Yr 3 were from a feeder pre-prep and DCs came from the second largest feeder pre-prep. DS1 up to that point and beyond was the one of my twins who was great at making friends. He was popular and happy, socially.
He made a new friend who'd come fresh to the school and was v involved with him that yr. Day 1 of Yr four, this new friend inexplicably 'dropped' DS1 - i think for all kinds of reasons like DS1 isn't into macho sports, that little boy - unusual for this school - came from a v v v rich background (literally country mansion etc and parents wealthy enough not to have to work ) and DS1 is from a single parent family with me struggling to pay school fees - but managing.
DS1 then was befriended by a big gang of children who'd been at the feeder pre-prep that has most children going to this prep. He had a great class teacher who helped friendships on their way (she then became head of pastoral care but is now on long term sick). I never worried re. DS1 and friends till this yr, Yr 5.
Each yr, the school remixes the three classes for each yr group. DS1 was with many of his 'friends' from last yr but for one reason or another, they'd drifted off. One friend from Yr 4 who's been very into drama and dance (like DS1) had dropped this in favour of macho football and rugby. A massive majority of the boys were and are obsessed with table tennis and this is where the problem really began.
A majority of those DS1 would have called friends last yr play constant table tennis on the school tables every single break. DS1 doesn't mind the occasional game but isn't good at this and isn't that interested. Most of the boys are v good at it and v into it. DS1 wanted to do other things but had no one to do those things with.
DS1 doesn't fit with the other group of boys - what you might call geeky swots - whereas this is the group his twin is in and he resents his brother sometimes walking round with what DS2 calls 'his friends' to talk about stuff of interest. (DS1 likes science like these boys do). DS2 has Asperger's traits that makes him fit with this similar group of children but also makes him v possessive of his 3 friends (he's happy to have only these three and that's it!)
DS1 was then left to wander round alone, as a few other boys do - including the child X ('bully'). Child X comes from the big feeder school cohort but has been more or less ostracised over years since age 3 by those children because of his behavioural issues and aggression.
So child X has told DS1 that he's his only friend - thus making DS1 feel special and wanted, as DS1 now feels he has no other friends.
But what can I, as a parent, do about this, other than continue to try to enable other friendships outside of school with his school mates? I've tried this and DS1 is v v reluctant to invite any back - although there's bene one boy - who's not in his class, where we've exchanged playdates a few times. This boy is fickle (as most may be at this age) with friendships and can be physically aggressive and is unusally tall and big - so towers above small, skinny DS1 hugely. He also love table tennis and football which DS1 doesn't.
They've now drifted apart a bit too. so DS1 is left with child X - who is incredibly charming, extremely clever and, were he an adult, would fit exactly the profile of a higly intelligent psychopath - in so far as he seems to enjoy controlling and hurting others, getting others into trouble and being extremely good at lying.
Long ago, pre-school, we used to mix with child X and his family when my sons were about 2 to 4 but I pulled away because the mum of child X felt that DS2's Asperger's traits would give a bad example to her 'perfect' son 
and also because child X got DS1, back then, to gang up against DS2 and got his little sister to do the same and the mum did nothing to prevent this or intervene - just blamed poor DS2. I also found child X's father's use of physical chastisement with chid X, his son, worrying and so stopped contact with the family.
I knew all about Child X well in advance therefore and from Yr 3 on have been worried that he'll get his grip on DS1 or bully DS2. He actually aggressed all the children in Yr 3, settled a bit in yr 4 but now scearly has his grip on DS1 and my worst fears are now realised.
Can the school really do anything practical about any of this? Without wanting to be genderist, the main teachers involved - DS1s form teacher (who actually doesn't teach DS1 for any lessons and barely knows him or sees him but supposedly coordinates his care) and the Head - are both male and don't seem v good at listening and understanding the isse. Form teacher is v v young and in his first teaching post and v inexperienced and I don't think has a clue about social subtleties. Headmaster is hugely unpopular with staff who've been leaving left right and cetnre since he started 4 yrs ago but is quite 'charming' with parents and excellent at 'selling the school'.
Incidentally, the theft was of a box of matches from the science lab. DS1 LOVES science and the science teacher and would never have contemplated stealing from anyone but especially not from her, on his own. DS1 knows to respect matches but has used them under close supervision at home with me, so they don't have a 'special 'forbidden fruit' aura around them. Child X has previously brought matches into school - which were confiscated, superglue and glued children's desks down in yr 3, stole a bike and rode off alone from school etc etc. Surely the school must realise that the idea of taking the matches was child x's?
What can I expect the teachers to do about any of this? There are so many children and so many v new teachers here. the head of pastoral care is on long term sick and the form teacher - who should be our first pot of call, is inexperienecd and rarely sees DS1 anyway. child X is in the same class and the school never swop children to different classes in the middle of the year.
I still feel v sad for DS1 and v worried and have been awake since 1.30am worrying about it all.