Second-level students may be left without teachers during the coming term.
The Teachers' Union of Ireland has ordered its members not to teach classes that are above a certain size.
The union has warned that it may take disciplinary action against teachers who disobey the instruction.
General Secretary Peter MacMenamin said the union policy was not new.
'Of course they won't be walking out and leaving students unattended,' Mr MacMenamin said.
He said if procedures are not followed, students will be escorted to a safe place where they can be supervised by a principal or deputy principal.
Mr MacMenamin said class sizes under the instruction of the Department of Education and Science are 30 for general subject classes and 24 for practical classes.
