Mary Lou McDonald says the Government needs to make clear whether public health authorities signed off on the decision to allow Leaving Cert students return to school.
The Sinn Féin leader has accused the Government of having a "wing and a prayer" strategy for introducing COVID-19 restrictions.
A range of new lockdown measures were announced yesterday, including that schools would remain closed for at least the rest of the month.
Senior ministers have defended the decision to allow Leaving Cert students to attend classes three days a week from Monday, insisting they followed NPHET advice.
On today's Newstalk Breakfast, the Health Minister Stephen Donnelly read extracts from the CMO's letter to him, including: “While the experience from September to December has clearly demonstrated that schools are in themselves a safe environment, the current epidemiological situation has deteriorated to a point where significant level of mobilities and linked activities that the full reopening of schools would generate constitutes a very significant additional risk.”
However, Deputy McDonald told The Pat Kenny Show that ministers need to say in the 'plainest terms' whether the Leaving Cert decision was a move signed off on by public health officials.
She said it's her 'understanding' that the Leaving Cert decision wasn't 'authorisied, cleared or advised upon' by NPHET.
She said: "I am very, very concerned that there is now a level of stress, concern and confusion among parents, students and teachers… because the Government has not set out in the plainest of terms the advice it got from NPHET and the Chief Medical Officer which told them it was safe.
“My first alarm is specifically that absence of very clear medical advice. And then secondly… the lack of preparation. Listening to some of the stakeholders yesterday… they indicated their meetings with the minister were very brief. That’s very troubling.
"I accept we’re in a very, very difficult situation, and any decisions that will be taken are tough decisions. What I cannot accept is the level of disorder and lack of planning from Government."
She suggested it's 'premature' to say thousands of students and teachers can go back safely without 'absolute assurances' from public health experts.
The opposition leader said she welcomes the Government's ability to introduce and announce necessary new restrictions, but is 'astonished' at the lack of planning for the consequences of those moves.
She argued: “On the issue of the Leaving Cert exam itself, I very much hope we can have a traditional Leaving Cert exam in the summer. But I know for that to happen there has to be extensive contingency planning by Government.
“I am very concerned we have had no word of that or seen no progress on that. That suggests to me that we have a strategy that is a bit a ‘wing and a prayer’ from Government. You can’t afford to adopt that position when things are so serious and so tough for people."
Elsewhere, the Sinn Féin leader welcomed the 'belated' introduction of new travel restrictions for those arriving from Britain and South Africa.
However, she said she doesn't know why it's limited to just those two countries.
She argued: "We need a proper system of not just an initial negative test, but then a second test when a person has landed on our shores.
"We need to have a proper system of checks and quarantine for people when they arrive."
Deputy McDonald reiterated her party's argument for a 'coherent all-Ireland approach' to the pandemic, including testing and tracking of anyone arriving on the island.
She conceded that such close cooperation between Dublin and Belfast was not possible - and acknowledged that the blame doesn't necessarily lie with the Government in Dublin.
She noted there has been a “considerable issue” in Northern Ireland when it comes to finding consensus with unionists on the way forward.