ASTI general secretary Charlie Lennon said teachers' campaign for a pay rise "will end when the Minister makes us an offer that we can’t refuse".
"That’s a matter for the Minister and the Government to determine. Our members are disengaging from an activity which they’ve engaged in over the years on a voluntary basis.
"They are saying they’re not prepared to do that anymore because of the failure to address their pay claim."
Marie Damaswamy from the National Parents Council described the situation as unfair on schoolchildren.
"It’s unfair to the students because they’re not getting the quality of education that they should get," she said.
"There seems to be no movement from the ASTI or the Minister. At the end of the day, it’s our children, our exam students, who are in danger."
Mary McQuill, of the Association of Principals and Deputy Principals, said she believes all schools will be able to open this morning, but some may have difficulties if the hired replacements have not been fully trained.
"Hopefully everything will go according to plan," she said.