What you said
Insist on some support from whoever is in charge of this year group. Log everything you have done and send it to this member of the management team. I have always found it more effective if I suggest what I would like them to do. Parents should be invited in and the boy should be present with you and a senior member of staff.
The expert view
You are right to focus on the one child - you may have three hard cases of equal density, but it is more likely that you have a leader and two henchmen.
The key for this boy will always be sanctions, applied fairly, consistently and with rigour. You say that you have given detentions, but how consistent have you been? Is he always given the same sanction for the same thing, or is he sometimes let off? He needs to know that your electric fence is always on.
If he gets a sanction and continues to misbehave, there must be an escalation: from a half-hour detention, to an hour, to a Saturday, up to and including exclusions. He needs to see that the more he swims against your tide of disapproval, the stronger the tide grows until it becomes untenable for him.
If you are not getting support, this is a big problem, because the last point only works when line management helps to make it happen. So speak to the people responsible and ask them what to do next, what they plan to do and when. Take a note of what they say, then if their word fails to match the world, call them up on it, politely. Ask them again: what happens next?
This boy may even be enjoying your discomfort. So keep cool, but be definite. If he acts up, he is sent out or removed, with no fuss, just professional determination.
It may take some time, but you can be the boss of this situation. You just need to reboot your expectations and rules a bit.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. http:behaviourguru.blogspot.com
Post your questions at www.tes.co.ukbehaviour.