Thousands of pupils are being told to stay at home next Wednesday due to a planned teachers' strike.
Many schools will close altogether and classes in some others will not run because of action by the National Education Union (NEU).
It is the first of four planned strike days over pay by teachers and support staff.
Discussions between unions and the Education Minister Jeremy Miles have so far failed to resolve the dispute.
At the same time, the National Association of Headteachers (NAHT) is beginning industrial action short of a strike, which includes only doing some tasks in core hours and refusing to cover striking staff.
NEU's senior policy officer, Mary van den Heuvel, said the impact of the strike would be a "mixed picture".
