The National Union of Teachers (NUT) stepped up its industrial action over pay, pensions and workload with action short of a strike today.
They joined members of NASUWT who are already taking action - which now means that nine out of every 10 teachers in England and Wales are involved.
Teachers are already reporting that bosses are trying to lean on them with veiled threats and intimidation.
Christine Blower NUT general secretary said: "Teachers are being undermined by a government whose almost daily criticisms and erosion of working conditions and pay, coming on top of previous attacks on pensions, are unacceptable.
"This negative approach to the profession has to stop.
"The action we are taking at this stage will have a positive benefit on children's education and restore a balance to the working lives of teachers."
NASUWT general secretary Chris Keates warned that action could increase later this year if the government does not respond to teachers' concerns.
The union would challenge any schools or employers penalising teachers for taking legitimate industrial action, she said.
"The action is already having an effect because there have been threats from local authorities and schools of breach of contract or deductions from salaries.
"The more that threats are made, the more it strengthens the resolve of teachers."
Ms Keates said action covered strategies such as cutting down on paperwork - but she warned the position could change, especially after a report from the teachers' pay review body later this month.
"Teacher morale is at an all-time low and our research shows that over half are seriously considering leaving the profession.
"We don't want privateers and marketeers in schools. We have a fight on our hands - and we cannot wait until 2015."