If membership of the BNP doesn't interfere with their work in the class room there is no case to ban it. The NASUWT believes that it would interfere with the job:
"Maurice Smith seems to have focused, to a point of obsession, on the number of incidents," she said. "One incident is one too many. How many incidents would there have to be before Maurice Smith would be persuaded that further action is needed?
"The idea that a person who signs up to membership of the BNP can simply leave these beliefs at the school gate and behave as a 'professional' when they walk into school is risible.
"A principled stand was required. This is a matter of social justice, staff wellbeing and child protection."
Well if the NASUWT believes that it is impossible for teachers to leave their beliefs at the school gate, then fine but if they really believe that behaving professionally is an impossible task for teachers then there are other implications that flow from that. It would make it very difficult to argue that members of a religion could be science teachers or that politically active socialists could be trusted to take history lessons.
The logic of the NASUWT position means that teachers require a much greater degree of supervision and less autonomy. However I have more faith in the teaching profession than their union does though. I'm sure they can act professionally and if they can't and any teachers, BNP or not, start talking about the myth of the Holocaust during History lessons then they can be sacked for that.
In fairness there are some organisations with a track record of trying to disrupt the education of children and promoting political extremism however on balance I don't think I would ban membership of the teaching unions.