The Association of School and College Leaders said the pandemic had caused massive disruption in England, and warned schools across the country were "not out of the woods yet".
Department for Education figures show pupils across Northumberland missed the equivalent of 1.1 million days of in-person education between January and the end of March for coronavirus related reasons – 57.8% of all possible school days.
In the autumn term, 170,590 days were missed for this reason, meaning youngsters were absent for 1.3 million school days over the two terms – equating to roughly 35 per pupil.
The figures include state-funded primary, secondary and special schools in the area.
Children across England were sent home to self-isolate when coronavirus cases were detected in the autumn.
But for the majority of the spring term, students – except children of key workers and vulnerable pupils – were told to learn remotely amid the national lockdown.
Around 252 million school days were missed nationally because of Covid-19 over both terms – 29.4%
Geoff Barton, general secretary of the ASCL, said: “Covid has caused widespread educational disruption across the country, but the impact hasn’t been uniform.
"Even when schools were fully open during the last academic year, attendance varied widely depending on local circumstances.
"Unfortunately, there is still a very high level of disruption taking place this term and we are not out of the woods yet."
He said the immediate priority for the government should be to end disruption by increasing the vaccination roll out for 12 to 15-year-olds and encouraging twice-weekly home-testing.
Schools record general absence – including when authorised and unauthorised – separately, with 3.3% of sessions missed during the spring term due to absence.
In Northumberland, the absence rate was 3.8%.
A DfE spokeswoman said the vaccination programme and adherence to public health advice has put schools in a better position than the last academic year.
She added: “We continue to work with parents and school and college staff to maximise students’ time in the classroom, and our long-term education recovery plan, supported by over £3 billion to date, will deliver world class teacher training and give millions of children access to high-quality tutoring.”