Figures from Northumberland County Council reveal that 5,799 pupils, which represents 13.5% of the total, were in the classroom on January 11.
This compares to 682 pupils who attended on May 26 last year, prior to the phased return of some primary classes in June.
However, the efforts of Northumberland school staff in coping with these additional numbers has been praised.
Cllr Guy Renner-Thompson, cabinet member with responsibility for children’s services at Northumberland County Council, said: “While there is additional pressure on our schools with more pupils attending this lockdown, the schools are doing an outstanding job in arranging themselves and making sure it's safe for staff and pupils.
"The county council is supporting schools 100% during this difficult time with support and guidance.”
The increase has been put down to a combination of factors, including the Government’s broad definition of critical workers.
It has led to calls for more measures encouraging parents to keep their children at home.
Dr Patrick Roach, General Secretary of the NASUWT, said: “Further steps are needed to strongly encourage parents to keep children at home and keep themselves and others safe.
“Reducing the spread of the virus in the community is critical if we are to ensure that all schools can remain open safely.
“Some schools are reporting that the majority of parents are demanding places for their children during the current restrictions. That means we have many more children and adults mixing in schools than would be expected under this national lockdown and the risk of the virus being transmitted to others on their journeys to and from schools and during the school day remains high.
“The new coronavirus variant is much more transmissible which means that much more needs to be done to ensure that the Government’s stay at home message is being followed.
“The Prime Minister has been very clear to the country that schools do act as vectors for coronavirus transmission and may cause the virus to spread between households.
“It is essential that Ministers do whatever it takes to ensure that parents can afford to keep their children at home and that children who need to do so are able to access their education safely from home.”