A teaching union has said the Education Minister must take the threat of coronavirus spreading in Northern Ireland's schools seriously.
It comes after officials admitted that additional funding will be needed from the Executive to ensure the safe return of pupils.
Schools are due to reopen on a phased basis from next month, and Education Minister Peter Weir has said a range of safety measures will be necessary.
However, the Department of Education has said it does not have the budget for Personal Protective Equipment (PPE) and other Covid-19 safety measures.
Justin McCamphill of teachers' union NASUWT said that if proper safety measures are not in place, the virus crisis could quickly "blow up" again - with schools the new frontline.
Earlier this week Mr Weir said the return of pupils for the new term is possible, even though it is only weeks away.
He has been facing growing pressure to set out a comprehensive plan detailing how a safe return to classrooms can be achieved.
However, the department has admitted that issues have arisen over funding.
A spokesperson said: "The Department and the Education Authority have worked together to quantify the anticipated costs to facilitate the reopening of schools, which includes PPE.
"As there is insufficient funding within the education budget to address the quantum of pressures identified, additional funding would need to be agreed by the Executive."
Mr Weir plans to bring a paper to the Executive to get agreement on the issue soon, however, it is likely that other departments will be bidding for funding too.
Mr McCamphill said that their members are "very concerned" by the lack of clarity on the situation as well as the lack of funding for schools.
He said: "They're also very concerned that Peter Weir seems to be disregarding his own guidance that he published on June 19, which set out the plan for how schools should reopen in August and September.
"When that guidance was introduced in June, the incidence rate was fairly low, we had hoped that would continue to fall and maybe if we were lucky it wouldn't be needed.
"But that's not the case, we have significantly more cases now than we had in June, therefore the Minister shouldn't be moving to substantially change the guidance that was drawn up.
"We clearly think the Minister of Education seems to operate under the illusion that schools are some sort of magical places where coronavirus can't be transmitted and he needs to take this seriously and ensure that teachers, other workers and pupils are fully protected when schools reopen.
"The guidance says that schools must do this, must provide the sanitation etc, but without providing money they are leaving schools in a very difficult position and schools are being left to decide how much to have in relation to PPE and sanitising equipment.
"But our view is that schools need to be allocated funding and there is a model for doing this - in the Republic they have applied a formula based on the number of pupils in every school that schools will be provided extra funding so that they can then purchase hand sanitiser and protective equipment."
Stormont ministers are being advised by Chief Medical Officer Michael McBride and Chief Scientific Advisor Professor Ian Young, who both said this week that "there would be some benefit" to pupils wearing masks at school.
Mr McCamphill added: "I think it would be rather bizarre if young people had to have masks in their pockets so that they could get on the school bus or go into a shop on the way home, but then wouldn't be allowed to wear the same mask in school.
"But if we want to educate the populace and young people to wear masks, education like with other life skills needs to be developed in school, so we need to model that practice of wearing masks in school.
"There is guidance but again, no money to back up that guidance, and that is what has got principals and teachers most frustrated, they've been told what to do but haven't been given the money to do it.
"Time absolutely is of the essence.
"While there will be a phased return, if we are to build up to a full return, if that's possible, we're going to have to make sure that all the correct health and safety measures are taken, otherwise this will all blow up and everyone will be off again.
"It is the wider population we want to protect, many of those young people will be going home to parents and grandparents who are in the vulnerable categories.
"We are calling on the Minister to think again about bringing proposals to the Executive for full reopening.
"The Minister needs to continue to work with trade unions on implementing the guidance that was put in place in June and ensuring that schools receive adequate funding to ensure the measures can be put in place to protect pupils and staff as well as the wider community."
Schools will resume in some way or other in four weeks' time and a lot of the focus in education has been on the arrangements that need to be put in place. The need to address pupil underachievement has also been prioritised by the Education Minister.