TEACHERS fear there will be a "catastrophic" surge in Covid-19 infections if schools are told to reopen too soon.
Union leaders have shared their concerns in a letter sent to education ministers in the north, Republic and Britain.
The British and Irish Group of Teacher Unions (BIGTU) represents almost one million education staff.
Some European countries including Denmark have begun a phased reopening of schools.
Their approach to ending the educational lockdown is being closely observed as other countries debate similar measures.
Prime Minister Boris Johnson indicated that primary schools would be prioritised for reopening when the lockdown is eased.
Some reports suggested that groups of pupils could return to classrooms as early as June 1 in England.
There is no target return date in Northern Ireland.
In the Republic, schools will remain closed until September at the earliest.
The letter from BIGTU was sent to minsters including Peter Weir and Joe McHugh in Stormont and Dublin.
Its signatories included Ulster Teachers' Union general secretary Jacquie White, Irish National Teachers' Organisation general secretary John Boyle and the leaders of the TUI and ASTI in the Republic, John Mac Gabhann and Kieran Christie.
"We wish to urge significant caution in any consideration of reopening schools," BIGTU wrote.
"Firstly, we would highlight the very real risk of creating a spike in the transmission of the virus by a premature opening of schools.
"We are convinced by the experience of other systems that a critical tool in preventing a surge of infection is an established capacity to `test, trace and isolate' and we would argue that reopening schools before such a regime is in place would be catastrophic to the rate of infection.
"Secondly, it is clear to all that schools can only reopen and operate safely if there are significant operational changes in place to ensure effective social distancing, strong hygiene routines linked to thorough cleansing practices, appropriate PPE available where required, and ongoing risk assessments in place to monitor operations.
"This will mean that as schools cannot reopen as normal, a phased return will be required and priorities established around attendance, which is likely to be part time for most pupils.
"As part of the establishing of priorities we would stress a third point – which is the need to consider equity.
"We recognise that children from poor and challenged backgrounds and vulnerable children will inevitably require significant additional support as we move slowly back to a more settled situation."
It added that potentially all children will have suffered a level of trauma as a result of the Covid-19 pandemic.
"We would urge that the initial focus when schools reopen, in any capacity, should be on the health, well-being, and emotional resilience of our students," BIGTU said.
