Reopening schools next month would be "wholly premature", a teachers' union has warned Education Minister Peter Weir.
NASUWT said teachers should have access to personal protective equipment, and those schools with significantly fewer staff available due to the coronavirus impact would need to ensure "stringent" social distancing.
The union's general secretary Dr Patrick Roach said: "The suggestion by some of an arbitrary date (eg June 1) for the reopening of schools is wholly premature.
"With the highest Covid-19 mortality rates in Europe, it is clear that government must continue to adopt an extremely cautious approach which does not contribute to further deaths and a further intensive wave of spread of the virus in the UK."
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It is "extremely likely" there could be a phased return in September, at the start of a new educational year, the minister told the BBC's Radio Foyle.
The union said it recognised the "absolute need" to maintain efforts to suppress the virus.
Dr Roach added: "In this context, the NASUWT is firmly of the view that changing the current restrictions on the opening of schools would be highly premature and extremely damaging.
"Schools must be reopened only when it is safe to do so.
"In view of the continued and pressing public health challenges and the considerable task that will be required to ensure that every school is ready to admit increased numbers of children and adults into safe learning and working environments, the NASUWT urges you to act to end speculation on the reopening of schools beyond the current restrictions prior to September 2020."
Mr Weir has said no decision had been made on when schools will reopen.
He said when they do it will unlikely be on a business-as-usual basis, with a phased return of pupils anticipated alongside the continuation of some remote learning.
Dr Roach said the minister should ensure the health and safety of those working in schools.
He added Covid-19 risk assessments must be undertaken appropriately and effectively in all settings prior to and during any phased reopening of schools, in consultation and agreement with the workforce and trade unions.
He wrote: "In addition, your department will need to set out its expectations in detail to ensure regular and effective cleaning and hygiene practices in schools prior to and following any reopening."
Around 470 schools are providing supervision for about 1,300 children whose parents are key workers involved in the Covid-19 response.
Schools in the Republic are due to reopen in September at the start of the new academic year.
