When the NUT announced its joint campaign of industrial action with the NASUWT, the union's deputy general secretary Kevin Courtney insisted: "We don't want there to be conflict with heads."
But now, a month after the protest over teachers' working conditions, pay and pensions started, evidence has emerged of serious tension between school leaders and teachers. In one school, Stratford School Academy in East London, the head's threat to dock the pay of teachers taking part by 15 per cent has already led to strike action.
According to John Dixon, head of the NUT's membership department, the union has issued about five more strike notices as a result of heads threatening to discipline teachers taking action. A senior figure in one of the headteacher unions said evidence suggested that about one in 10 schools was experiencing significant internal strife.
The action has seen 25 instructions issued to NUT and NASUWT members, on issues including appraisals, lesson observations and attendance at meetings. Under the action, which the unions insist is "pupil, parent and public friendly", teachers are also instructed not to submit lesson plans to senior management, not to send emails outside of their directed working hours and not to cooperate with practice inspections.
Malcolm Trobe, deputy general secretary of the Association of School and College Leaders, said that while many schools had been unaffected, some had reported major problems arising from the action.
"In schools where there has been an extremely strict interpretation of the action, it is causing some disruption and is making it difficult for heads to run things as they want to," he said. "What we want to know is: what would lead to a cessation of the action? It is not clear what (the unions) want to achieve. We will be meeting with the secretary of state and asking what he is doing to resolve the situation."
Russell Hobby, general secretary of the NAHT heads' union, said the impact of the action on schools was "patchy". "We are seeing in a few schools teachers in middle leader roles who are refusing to take part in the observation of colleagues and not getting involved in their mentoring and development programmes," he said. "Technically, of course, they can do that, but it's a shame to see.
"There are local flare-ups rather than a consistent pattern. It seems to be dependent very much on relationships with reps at a local level and what attitude the reps are taking. For most, it has not changed what's going on in school."
The current action by the unions is the latest phase of a lengthy industrial campaign. Both teaching unions went on strike over the reform of teachers' pensions a year ago, before rejecting the government's final offer. While ministers insisted they would impose the deal anyway, the unions have widened their joint strategy to include teachers' pay and working conditions, but have not yet announced any plans for a national strike.
Mr Dixon said that while "four or five" notices of strike action had been sent to heads, they had all been suspended "where progress has been made".
"We think the action has been quite successful," he said. "It was designed to implement what we think the secretary of state should be doing through regulation. We are picking up that, really, it isn't causing problems for schools at all, and is allowing teachers to concentrate on the activities of teaching and learning.
"We have some schools that have perhaps drifted into putting unacceptable demands on staff, but these are the exceptions rather than the rule. A lot of schools have just fallen into line."
From the forums
(The action) has not gone over well with the head and deputy head and it has created a horrible situation at the school. They are furious. Not sure if excessive workload or senior management that resents you is worse.
My head and deputy head told the staff that they support the action, understand who it's aimed at and think it's justified.
The fact that most of my colleagues either don't give a toss about why the unions have proposed (this) action, don't want to work to rule and are carrying on as normal is driving me mad!
I have said no to cover! You have to have a little courage, and it was a bit uncomfortable for me. but I said I was following union instructions.
In a village primary, if I decided to work to rule, the people that would bear the brunt are my colleagues and the children I teach.
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- Original print headline: Industrial action sparks clash of heads and teachers