The president of the Secondary Principals Association, Deidre Shea, says Tuesday's strikes will be disruptive, but it understands what the teachers are trying to achieve.
Thousands of Year 9 pupils are expected to miss school as secondary teachers start their five weeks of industrial action.
Members of the Post Primary Teachers Association won't teach one year group of students each Tuesday for four weeks, starting with Year 9 students.
Ms Shea told Kate Gudsell most principals are expecting the Year 9 students to stay away from school.
