The Teachers Union of Ireland has reportedly ordered all of its members not to teach classes above a certain size.
Reports this morning say the union has sent a letter to teachers in 300 secondary schools warning them that "deviation from the directive will not be tolerated".
The TUI said: "At no time are teachers instructed to walk out of classes or to leave students unattended. This would be wrong, unprofessional and irresponsible."
The union added the instruction to teachers follows a 30-year-old policy designed to " protect the education quality for the students".
This morning's reports say teachers could have their pay docked if they refuse to teach a class, with school managers warning that such action could result in pupils being sent home.