More than a third of school leaders are experiencing staff absence levels of over 10% due to Covid-19 related reasons, a survey suggests.

A majority (95%) have pupils off for Covid-19-related reasons at the start of term – and 29% said they had more than 10% of their students absent, according to the snap poll by school leaders’ union NAHT.

Paul Whiteman, general secretary of the NAHT, has warned that schools are “teetering on the edge” with “considerable” shortages, adding that absences due to illness may “very likely rise” during the term.

The findings came as pupils are returning to class this week after the Christmas break, with new advice for secondary school and college students in England to wear face coverings in classrooms.

Education Secretary Nadhim Zahawi has admitted that staff absences are likely to rise in schools in the weeks ahead with increasing Covid-19 cases.

Last month, the minister called on former teachers to return to the classroom as part of efforts to tackle staff shortages.

A poll, of nearly 2,000 school leaders in England, suggests that half are using supply teachers to cover absences.

But among the school leaders who have needed to use supply teachers, 37% said they have been unable to source the supply staff they need.

The survey suggests that 36% said they had more than 10% of their total school staff absent on the first day of term due to Covid-19-related reasons, while 27% said they had more than 10% of their teaching staff off.

Nearly one in 10 (9%) reported that more than a fifth of their teaching staff were absent, according to the poll of NAHT members on Wednesday evening.

Ahead of the start of term, the Department for Education (DfE) told headteachers they may want to consider “combining classes” in the event of staff shortages to keep face-to-face teaching in place.

But a coalition of five unions representing teachers and support staff has suggested that schools which routinely ask staff to teach more pupils in merged classes amid shortages should be challenged.

A “safety checklist” – from the National Education Union (NEU), NASUWT teaching union, Unison, GMB and Unite – says merging classes “should not be adopted” as it will “increase virus transmission”.

Teachers expected to routinely accept extra pupils from combining classes should “urgently” raise it with their union, the advice says.

It comes after Mr Zahawi told heads to consider merging classes, or sending groups of children home, if shortages reach critical levels.

The poll suggests that only 7% of school leaders have combined classes or pupil groups, and only 4% have had to send some classes or groups home.

Mr Whiteman said: “Schools are doing everything they can to maintain education for pupils in the face of very challenging circumstances.

“Staff absence on the first day of term was considerable and school leaders have been doing what they can to redeploy teachers and other support staff to avoid being forced to combine classes or send groups home.

“Given that this is a snapshot of just the first day of term, this is a very worrying picture. Infection rates – and therefore absence due to illness – could very likely rise as the term progresses, and already half of schools are having to turn to supply staff, with many finding they cannot secure suitable cover.”

He added: “Many schools are teetering on the edge and the next few weeks at least will undoubtedly continue to be an incredibly challenging time.

“School leaders are having to make difficult decisions whilst faced with conflicting priorities. The Government’s clear priority that schools should be kept open is understood.

“The crisis in available resources will mean that school leaders and their teams will require the unflinching support of government to achieve this.”

A Department for Education spokesperson said: “Schools across the country reopened this week and staff are working tirelessly to ensure classrooms are safe for face-to-face learning, and despite the challenges in this first week of term, millions of pupils have returned to be with their friends and teachers.

“We’ve supported schools to continue classroom learning for pupils through encouraging former teachers to step in, and extending the Covid workforce fund for schools that are facing the greatest staffing and funding pressures.

“We’ve also asked schools to have contingency plans to maximise attendance and minimise disruption to learning, should they have high rates of staff absence, and are working with the sector to share case studies of flexible learning models to support the development of those plans.”