The findings of a review into a school campus at the centre of health and safety concerns will be published later.
Water and soil testing was carried out at the St Ambrose and Buchanan High School campus in Coatbridge, North Lanarkshire, as part of a Scottish Government-ordered independent review.
Parents have spoken of blue water coming from taps at the schools, built on a former landfill site, and have expressed fears of a link between possible exposure to chemicals and illness among staff and pupils.
Results of the tests will be published on Friday ahead of the schools reopening on August 12.
The NASUWT teachers’ union said that unless members are satisfied the site poses no health risks, they will not return to work on Monday.
General secretary Chris Keates said: “Our members’ summer break has been blighted by the serious stress and anxiety of the concerns about the risks to their own and their colleagues’ health.
“Pending the outcome of the report, the union has already put contingency plans in place to continue to protect members.
“The council has been notified of a period of continuous strike action from the first day of term.
“This will undoubtedly go ahead if the NASUWT is not satisfied with the report and the council proposes to open the schools as usual.”