i read that as he was sent to the time out chair because he was asked to sit quietly and failed to do so, even after a warning. (not that he was sent to the time out chair for an un-named offence and then did not sit quietly etc etc)
so i think the not-sitting-quietly was the offence, not a continued description iyswim.
it sounds like the sort of thing which you would have signed up to at the beginning of the year tbh 'students are required to behave in the following manner. if they fail to do so they will be asked to correct their behaviour. if they still fail to do so they will be a) sat on the naughty chair b) a letter will be sent to parents. if behaviour persists parents will be invited into school to discuss with teacher. ' etc etc ad infinitum. it'll be listed as a Good Thing in school policy - ie we work closely with parents to manage difficult behaviour etc etc
although if any of mine had come home with a letter like that, i'd have said 'why weren't you sitting quietly?' and given them rock all.
but like i said earlier - it is his behaviour that is causing the problem - he has been assessed and the ed psych (presumably) has come up with some guidelines which seem to be yielding some results in some areas. you need another meeting with the senco and teacher/ ht to discuss behaviour, as the esteem guidelines don't seem to be improving his ability to behave in an appropriate manner in class.
(i'm not being critical btw - it's never great to acknowledge your child has behaviour issues - but i'm a firm supporter of those with additional needs of whatever sort (including behavioural) getting the help they need to learn coping strategies. for their own education's sake, and the sake of their peers.
like alouiseg says, the next step is other parents going into the school saying 'we are worried about our children's education because it is being continuously disrupted by weaslet. (my answer usually is to co-opt these parents and ask them to go in and complain so that you have additional ammunition for getting your son the help he needs etc etc.) but i have to say that's usually reserved for slightly more extreme behavioural difficulties than talking in class lol
but he does need some more help lol.