Teachers in the NAS/UWT union are threatening national industrial action in a campaign to cut their working hours.
The union said two thirds of its members believed they did not have the right "work-life balance", despite Government reforms aimed at freeing up their time.
The union suggested that teachers across the country could be balloted on whether to stage a work-to-rule if their workload does not ease.
Delegates at the union's annual conference in Birmingham will debate calls to "support industrial action, where appropriate, to secure an acceptable work-life balance for all teachers".
Under a deal struck with the Government, teachers should have half a day off every week for planning lessons and marking, while an army of classroom assistants have been hired to complete administrative tasks.
NAS/UWT general secretary Chris Keates accused head teachers of "staggeringly unlawful behaviour" by flouting the rules.
She continued: "We may have to enforce these provisions ourselves.
"If we can't get satisfactory provision by Government, we may have to ballot members nationally so they work to the agreements."
The debate follows a survey by the union that found 64% of teachers did not feel they had a reasonable work-life balance.
The research found there was "no limit" to the number of meetings teachers are asked to attend every week. They also complained they were routinely called upon to provide cover for colleagues beyond their contracted hours.
