The Department of Education has confirmed that 19 out of 42 schools built by Western Building Systems will now need external safety measures put in place before they can reopen.
It comes after the final structural reports in relation to the schools were analysed by engineers and department officials.
Contractors are starting the job of installing protective decking and temporary fencing at 19 schools to ensure the safety of students and staff.
19 other schools have been cleared to re-open without any remedial work.
The department has already announced that three schools will partially re-open, while remedial work is carried out and one building on the campus at Ardgillan Community College will be closed.
Minister for Education Joe McHugh says his department has taken a cautious approach to ensure school safety, and he hopes all the precautionary measures will be in place by Monday.
"Right throughout this process, our overriding priority has been the safety of school students and staff," he said.
"In cases where precautionary external or internal interventions are being implemented, this is with the purpose of ensuring the safe occupation of all or part of the building, and ensuring that disruption to classes is minimised.
"The next phase of the process will involve more detailed assessment and, where required, permanent remediation works to fully rectify any issues arising,” he said.
Western Building systems has responded by saying all its buildings were certified by the Department, and it says the work didn't involve any "shorts cuts or penny-pinching."
"Hard work, innovation and high standards have underpinned our business for 35 years," a spokesperson said.
"The Department itself awarded us contracts to build 42 schools over a 14-year period based on continuous quality of delivery.
"We honour our contracts, as we have always done. We recognise the responsibility we have to our clients. We know how important schools are to pupils, parents, teachers and the wider community."
You can find the summary of structural assessments here.