They have been derided by the education secretary as "Trots" and "enemies of promise". But if Michael Gove had been hoping that his assault on the teaching unions would fatally wound his ideological arch-enemies, he will be disappointed.
The classroom unions have seen their combined membership swell by more than 37,000, according to the latest official membership figures. At the same time, the number of teachers in England has dropped by 10,000. This means that during last year's national strikes, the unions managed to strengthen their grip on the teaching profession, despite it already being the most unionised part of the UK workforce.
By the end of last year, the NUT had gained 15,768 new paid-up members in 12 months. The NASUWT teaching union saw its roll grow by 10,965, while membership of the ATL rose by 9,402. Even Voice, the only union that completely refuses to take strike action, gained an additional 1,633 new teacher members.
The unions welcomed the figures, published by the certification officer last month, with predictable pleasure. "This is a clear indication that teachers are thoroughly tired of the government's continual attacks on the teaching profession," said John Dixon, head of the NUT's organising and membership department.
Professor Howard Stevenson, deputy head of the University of Lincoln's Centre for Educational Research and Development, suggested that last year's industrial action and imminent changes to teachers' pay and conditions could be contributory factors. "It's possible that people are seeing a number of things happening, feel a bit more exposed and want a bit more protection," he said. "There are issues out there that could be encouraging people into membership."
There has been little change in the power dynamic between the unions: 42 per cent of teachers who are members of a union are in the NUT, with the NASUWT just behind on 38 per cent. The ATL has 17 per cent and 3 per cent are members of Voice.
And there has been little movement in terms of market share since the end of 2010, but the ATL has seen the biggest gain, increasing by 0.41 per cent.
The figures should be treated with some caution, however: the unions use different criteria to calculate their total paid-up members. The NUT and the NASUWT both claim to be the country's "largest teachers' union" and are sceptical about one another's membership numbers.
While the figures do not technically include student, life and retired members, who do not pay full fees and are counted separately, they are widely believed by union insiders to be artificially inflated.
By way of comparison, the NUT balloted 218,370 members last year over changes to teachers' pensions - a considerably lower number than the 324,367-strong membership submitted to the certification officer.
Separate figures released in April this year revealed that the number of teachers in England dropped from 448,000 in 2010 to 438,000 in 2011.
The increase in union membership will come as a welcome boost to the union movement at a time when it is facing huge uncertainty.
A government consultation on the future of facility time, used by union representatives to complete casework and other union duties, began last month. The consultation document produced by the Cabinet Office said the government was looking to "find a balance between supporting constructive engagement with employee representatives...and providing better value for the taxpayer".
Currently most local authorities foot the bill for facility time, but last year Prime Minister David Cameron argued that the system "cannot be sustained, either morally or economically".
The Trades Union Congress claimed that facility time saves the public purse up to #163;586 million a year.
Paid-up members of the education unions: