Boris Johnson has declined to guarantee that all children in England will be back in classrooms before the summer holidays.

But the Prime Minister said he is full of “optimism and fundamental hope” that things will be different in the spring.

All pupils – except children of key workers and vulnerable pupils – have moved to remote education until February half-term amid tighter restrictions.

Asked to guarantee that all pupils will return to class before the summer break, Mr Johnson told the Downing Street press conference: “We think that with the vaccination programme we can do a huge amount to take out of the path of the virus those who are most vulnerable. That clearly offers opportunities to our country to do things differently.”

His comments came as school leaders said they were expecting a high turnout of children of key workers and vulnerable pupils onsite amid the national lockdown in England – with one school expecting hundreds to attend.

Steve Chalke, founder of Oasis Charitable Trust – which has 53 schools across England, said heads were preparing for a greater number of pupils to turn up to school on Wednesday than in the lockdown in March as more children were classed as vulnerable and more parents who were key workers wanted a place.

He said around a third of the cohort were expected to attend Oasis Academy South Bank, a secondary school in London, following the Government’s announcement to stay at home.

The Government guidance now says vulnerable children may include “pupils who may have difficulty engaging with remote education at home (for example due to a lack of devices or quiet space to study)”.

On the guidance, Mr Chalke told the PA news agency: “If you look at the criteria for vulnerable children, it has grown in several ways.”

He said: “So we are expecting more back this time because people trust us and trust the provision we’ve made. They trust us to make these spaces as Covid-19 safe as possible. And of course, parents aren’t in a position to be able to teach their children.

“Last time we had a lockdown it was the spring and the world was bright because it was the spring and you could take your children out.

“This time it’s freezing cold and you’re locked in and parents think well my child will be in a place where they’re actually learning, and with their peers and be safe.”

Some (schools) are saying - particularly in the more deprived areas - if all the children took up their offer of a place they might end up with 50%, 60% or 70% of their students onsite

Some school leaders could see up to 70% of their pupils in class, if all eligible children attend, prompting concerns about social distancing, staff shortages and the ability to balance remote education with in-person teaching.

The Association of School and College Leaders (ASCL) was also hearing from headteachers that more parents who were classed as critical workers were wanting to take up places during the new national lockdown.

Julie McCulloch, director of policy at ASCL, told PA: “The kind of indication that we’re getting is that more key workers are asking to take up those places than happened in the first lockdown.

“Obviously, schools are wanting to do what they can for those children and the vulnerable children as well, but some concerns that we’re starting to hear from some of our members are about the number of pupils they might end up with onsite.

“Some are saying – particularly in the more deprived areas – if all the children took up their offer of a place they might end up with 50%, 60% or 70% of their students onsite.”

With the Government acknowledging that exams in England will not be able to go ahead as planned in the summer, Education Secretary Gavin Williamson will update the Commons on Wednesday on how pupils will be assessed.

Mr Johnson said that Mr Williamson will work with exams regulator Ofqual to put in place “alternative arrangements” for the summer.

But despite facing calls to cancel this month’s Btec exams in light of the lockdown, the Government has left it to school and college leaders to decide whether they want to go ahead with the vocational exam series.

The decision came after ministers faced calls to cancel the January exams.