Thank you everyone for your replies.
We are now back from swimming and the first thing I did was sit him down to write a letter of apology which he has now done, very remorsefully.
It happened right before home time, the teacher was instantly reduced to tears (away from the children) and someone else had to cover for her for the last ten minutes of lessons.
Have now got whole story. He was encouraged by a friend to try to pull down a reception girl's skirt . The teacher asked him to stop and come inside with her, he didn't, she asked again, he ignored her, she went over to him, he lashed out and hurt her arm.
Madame, thank you, I have removed all privilages apart from a birthday party we have already RSVP'd to (no point punishing the other child as well) for one week.
He does have special needs, but is baffling the school as it isn't anything they can label like ASD.
He doesn't behave like this at home apart form the occasional outburst that is normal for children of this age. He is genuinely an easy child at home.
Giorine you are right, the no eye contact and stone-walling is definately shame, his eyes where also welled up with tears although he held them off as he seems to be unable to show emotion in public
He is quite shameful still now at home and keeps saying sorry and trying to get my affections back and getting reassurance that I still love him etc (obviously I have reassured him, just explained that I really don't like his behaviour)