The two hight schools, Buchanan and St Ambrose in Coatbridge, share a campus built on land formerly used for the disposal of waste including lead and arsenic.
Safety fears were raised last year when blue-tinted water was discovered coming from taps, and more recently it emerged that four current or former teachers at one of the schools had cancer.
The local council and medical experts have insisted the site is safe, but not all parents have been persuaded and 23 children have now been formally withdrawn.
One parent whose son is in P7 and is due to start at the campus in the autumn told the broadcaster she would not let him go if she was not certain it was safe.
“Many others will not go near the school until we have solid answers on this. There’s 200-odd kids leaving primary 7 going up to the school and they don’t even know if they’ll be going,” she added.
More than 16,000 people have signed a petition calling for staff and pupils at the schools to be tested for toxins, with some parents paying up to £400 privately rather than waiting for the NHS.
The Scottish Government has ordered an independent review in an effort to reassure parents, which Nicola Sturgeon has promised will “get to the heart of the issues” about the site’s safety.
Teachers at Buchanan High are currently in the middle of seven days of strike action and today they will be joined by colleagues at St Ambrose.
Chris Keates, general secretary of the NASUWT union, said the teachers were angry at the “continued failure” of North Lanarkshire Council to take action over the safety concerns.
The union is demanded a full site survey – including air, water, soil and tests on the fabric of the building – to be carried out immediately, arguing the matter is too urgent to wait for the review.
Gerard McLaughlin, head of education at North Lanarkshire Council, said there was “no evidence to support a link between blue water at the school and any serious ill health”.
“The school remains open to pupils, and teaching and learning will be maintained until the end of the session on Friday,” he added. “We welcome the support of the Scottish Government in providing further reassurance to families, staff and the local community.”