Some of Sunderland's schools are as full as they were before Christmas, despite the latest national coronavirus lockdown, councillors have been told.
Under the latest measures, pupils have been told to learn at home until at least February.
However, schools have remained open for the children of critical workers, or those deemed to be vulnerable.
A broadening of the definitions has meant that some primary schools could have almost all children attending.
Sunderland City Council's director, Jill Colbert, was speaking at a meeting of its Children, Education and Skills Scrutiny Committee, as reported by the Local Democracy Reporting Service (LDRS).
She said: "The Department for Education has significantly broadened the criteria of a vulnerable child to include any child without access to a device and any child which does not have a quiet space to work in.
"Arguably, given some of the housing environments some of our children live in, in some of the [city] wards that could be all children attending a primary school."
'People are scared'
Pupil numbers were further increased by the expansion of the criteria for "critical workers", which now includes those in sectors such as utilities, transport and finance, as well as health and social care workers.
Ann Blakey, headteacher at St Benet's RC Primary School, said: "We're public servants, we want to support critical workers, but if you look at the [critical worker] definition now it's harder to find someone who doesn't fit the list than who does.
"People are scared, mass gatherings are cancelled everywhere but I can have a full school of about 200-220 people.
"That would worry any community in this current situation, but we're not shying away, we're just worried."