Kerry George, senior assistant secretary of the National Association of Head Teachers, suggested the move as a last resort at schools in Wales, which missed out on English discussions about extra money for the workload reforms.
She told Welsh heads to embrace workforce "remodelling", saying failure to do so could create recruitment problems as teachers left for a better deal across the border in England.
But, she said, funding was vital: "The last thing we want is heads and governors at loggerheads with unions in schools because the money isn't there," she told NAHT Wales' annual conference in the Vale of Glamorgan.
"If you come to the point where it looks like it is absolutely impossible to implement the agreement in your school, I would say join the picket line with your staff."
From this term, schools are contractually required to relieve teachers of more than 20 administrative tasks, such as photocopying and chasing absent pupils.
In England, the London-based National Remodelling Team this week started training education authority staff to work with schools to implement the agreement in place.
But Mrs George told members of NAHT Wales that less was being done to help them: "If you have not got a recruitment problem yet, believe me, English schools will create one for you: already schools are starting to advertise for teachers saying 'we have implemented contract changes and have 10 per cent non-contact time'. " Welsh education minister Jane Davidson rejected claims that Welsh schools may be thousands of pounds poorer than in England (see TES, September 12).
She insisted Wales is matching spending in England and said comparisons were difficult because of differences in the way cash is delivered to schools. She said: "This year, education spending went up on average by 10 per cent across Wales. We are matching spending in England."
In Wales, money is distributed via the 22 local education authorities.
Heads will have input into funding decisions via new forums in each council area.
Chris Howard, president of NAHT Wales, told the minister heads had long-standing concerns about how money was passed on to schools and the transparency of the process.