Requirement is to give 7 days to the employer of intention to take industrial action. I assume they have done that. However there is no requirement to sat WHO is going to take the action, so in this case they don't know which teachers will be in or out and therefore which classes they can hold, if any. Easy thing would be to close the school but I guess they are trying to be helpful by not deciding until the day when they see which teachers turn up. If I were a parent I would be lining up my cover arrangements now or just plan not to send my child that day.