Just over half of eligible schools in England opened to more year groups in line with government plans last week, new figures show.
And the number of teachers working in schools rose to 183,000 on 4 June – up from 125,000 on 21 May, the week before the half-term break.
New data from the Department for Education shows that around half (52 per cent) of schools that normally accept at least one of the eligible year groups were estimated to be open to children in nursery, Reception, Year 1 or Year 6 on 4 June.
The figures were collected as children in England began returning in a phased process from 1 June, with Reception, Year 1 and Year 6 pupils heading back first.
Approximately 659,000 children attended a school on 4 June, representing 6.9 per cent of pupils who normally attend.
This represented a significant increase from 21 May, when around 244,000 (2.6 per cent) were in attendance.
Approximately 91 per cent of schools were open in some capacity on 4 June, up from 80 per cent on 21 May.
Under government plans, secondary schools will start to reopen to a wider selection of students from 15 June.
Education secretary Gavin Williamson said: "Last week marked the first, cautious step in a phased approach to bringing more children back into the classroom with their friends and teachers again.
"It is encouraging to see the majority of primary schools open their doors to more pupils, and almost double the number of children now attending early years settings.
"This is still a difficult time for families and many feel anxious about their children going back – but I can reassure families, and those working in education settings, that the welfare of children and staff will continue to be at the heart of all our decisions.
"Families should also be reassured by the incredible work teachers and support staff are doing to adapt their settings and routines, while making sure schools and nurseries remain as welcoming as they have always been."