sapphire, I've just caught up with this thread. What a worry for you. I agree, there seems to have been lots of mismanagement of the situation by the school. I too have heard that new legislation is coming in and teachers face the sack if they ignore possible signs of child abuse. I agree with Khara, this opens the way for a lot of misunderstanding.
When the teachers say they think the 'influence' comes from home, do they mean they suspect 'abuse'? It seems they are not communicating clearly to you. I think their tone and approach is judgemental and over the top, even if this is not the intention.
Is the school seeing the parents of the other children who also initiated this behaviour? You say it started in another nursery class and a whole group of boys were involved. Then your son was moved to a new class. Couldn't the major 'influence' have come from this group of boys and that your son has just carried on with this experimental play? He's so young anyway - it might not have sunk in that it's time to stop. As you say, it could well be a way of getting attention. Your son has told you another boy did it to him first. At worst, he or another of his playmates could be an abused child - has the school investigated this? Are they taking your son's words seriously, too?
I'd want to ask the school how often they encounter situations like yours and if they always call in the social services and school psychologist. If not, why this time? What happened was not desirable, but considering the ages of the children, surely not unique.
I think you should ask the head if the other parents are putting lots of pressure on the school (you say they have talked) and the teachers fear an official complaint if they do not comply. If you suspect that parental pressure is largely behind this, perhaps it's time to say you will also consider launching a complaint and will be keeping a diary of all conversations.
Have you also thought of getting the social worker to contact the school direct to say that they have found no evidence of abuse, or is this what happens anyway. Definitely worth knowing. And is it possible for the social worker to meet the head with you and mediate? This might clarify things all round.
Good luck