Coronavirus: Reopening schools is 'biggest post-devolution challenge'
Fully reopening schools in September has been described as "the biggest challenge" to face education in Wales since the creation of the assembly.
Gareth Evans, a leading academic, warned the pandemic's impact on pupils' learning had been "hugely significant".
Children are more likely to be harmed by not returning to school next month than if they catch coronavirus, the UK's chief medical adviser has said.
Pupils can return to school when Wales' autumn term starts on 1 September.
Education Minister Kirsty Williams has encouraged schools to accommodate all pupils in the classroom as early as possible.
However, the Welsh government has allowed a period of flexibility of up to two weeks so schools can make individual arrangements.
Mr Evans, director of education policy at the University of Wales Trinity Saint David, said it was "imperative" pupils returned "as quickly and safely as possible".
"It is absolutely clear the impact on learners has been hugely significant over the past few months and the attainment gap has grown significantly," he added.
"So we know that learners from more deprived backgrounds are missing out even more than their more affluent peers."
He said the recent controversy over exam grades had "been very significant and stolen much of the limelight," it was "important that we now refocus our energies into getting children back into school".
He added: "It's a huge challenge for our education system - and potentially the biggest challenge facing education in Wales since devolution."
The UK's chief medical officer Prof Chris Whitty said the chances of children dying from Covid-19 were "incredibly small" but missing lessons "damages children in the long run".
Rebecca Williams, deputy general secretary of the UCAC teaching union, supports that view.
"It's time for schools to reopen their doors because it's been a long time and teachers are very concerned about their pupils," she said.
"Not only for their education, but also their welfare. But it needs to be seen very much within its context.
"Schools that are reopening need to be doing so within communities where the transmission rates are at a reasonable level and not going in the wrong direction, but we also have to be making sure the school environment itself is as safe as possible to reduce all risks."
She said it had been a "difficult summer" for school leaders as they made the necessary preparations while also dealing with the fallout of the "qualifications debacle".
One headteacher said that while it was important to be mindful of the risk from Covid-19, the impact of lockdown on young people's mental health also had to be taken into account.
Owain Gethin Davies, head teacher of Ysgol Dyffryn Conwy in Llanrwst, said: "Seeing their friends and teachers and coming back into the school community is absolutely essential for them.
"Schools had learned lessons from a brief few weeks before the summer holidays when they were able to reopen in a limited way to allow pupils to check in and adjust to the 'new normal'.
"Now everybody's returning we've had to run new risk assessments and make a lot of preparations, including forming class bubble groups and being very mindful of periods such as breaktimes and so on.
"A lot is going on to make sure learners and the whole school community is safe."