Teachers and parents need "greater clarity" on the reopening of schools amid a rise in coronavirus cases, the head of a leading teachers' union says.
Patrick Roach, general secretary of the NASUWT, said teachers need extra details to prepare for pupils' return.
The government said it has set out the measures that schools in England should follow to reduce the transmission risk.
It comes as two scientists advising the government said some restrictions may need to come back into force to allow pupils back into the classroom.
And England's chief medical officer Prof Chris Whitty warned the country is "near the limit" for opening up society.
Mr Roach told the Observer ministers will have to convince staff and parents that it is still safe to reopen schools next month.
"The warning from the chief medical officer that a fine balance has to be struck in ensuring public health at this stage of the pandemic, and that the country may have reached the limits to the easing of lockdown, will no doubt prompt questions for many parents as well as for those working in schools," he said.
Boris Johnson has previously pledged that both primary and secondary schools in England will return in September "with full attendance".
Mr Roach warned that, if schools are to reopen safely, the government needed to give teachers clarification around the latest scientific advice "as well as sufficient time to review and, if necessary, adjust their reopening plans".
The National Education Union also issued a statement, saying the government needs "to monitor the situation nationally and in each region" and "be transparent about what the picture means for schools".
"It is clear, however, that [the] government needs a plan B in the event that restrictions have to be increased in or before September," said the union's deputy general secretary Avis Gilmore.
On Sunday, Housing Secretary Robert Jenrick told Times Radio that schools would definitely return to full capacity in September.
"I think you're right to say that reopening schools and getting our children back into the classroom with that direct face-to-face contact with their teachers will be a priority for the government when we have to make those tough choices," he said.
Prof Graham Medley, a scientist advising the government, told the BBC on Saturday that pubs or "other activities" in England may need to close to allow schools to reopen next month.
Prof Medley, chairman of the Scientific Advisory Group for Emergencies (Sage) sub-group on pandemic modelling, said he believed most people "think that opening schools is a priority for the health and wellbeing of children and that when we do that we are going to reconnect lots of households".
"And so actually, closing some of the other networks, some of the other activities may well be required to enable us to open schools," he added.
"It might come down to a question of which do you trade off against each other, and then that's a matter of prioritising. Do we think pubs are more important than schools?"
Another Sage member, Prof Calum Semple, from the University of Liverpool, said there would probably be a second wave of the virus in October and "some hard decisions will need to be made about what restrictions need to be reintroduced".
A Department for Education spokesman said: "We have set out the controls schools should use, including cleaning and hygiene measures, to substantially reduce the risk of transmission of the virus when they open to all children from September."
Measures due to come in this weekend, including the reopening of casinos, bowling alleys, skating rinks and some close-contact services, as well as the return of indoor performances and pilots of large gatherings in sports venues and conference centres, have been postponed for at least a fortnight.
The expansion of wedding receptions to allow up to 30 people is also on hold.
Mr Johnson said on Friday he needed to "squeeze the brake pedal" on easing restrictions following a rise in coronavirus cases.
Latest figures showed a further 74 deaths were reported in the UK on Saturday, taking the total number of people who have died after testing positive for the virus to 46,193. The latest government statistics also showed 771 new cases had been confirmed.