Parents of the 910 students at Holywells high have been told that all year groups, except 10 and 11, will be asked to miss out on at least some of their weekly lessons.
Staff in the NUT and NASUWT have voted not to cover for five positions that have been vacant since Christmas. From Monday, some classes will e sent home each day, after morning break or at lunchtime.
Worst hit will be Years 7, 8 and 9 who will all miss six of their 40 lessons per week.
Head Barry Whelpton said: "Parents are naturally concerned but they seem to understand why teachers are taking this action and so do I."
Mr Whelpton said he hoped the arrangements would not last beyond Easter when an agency that recruits from South Africa has promised to help plug the gaps.