Headteachers say they are having to spend significantly more time in the classroom covering self-isolating teachers this term – at a time when their workload is already at an all-time high owing to Covid-19.
The NAHT school leaders' union has said heads are being forced to step in due to a lack of funding for supply teachers to cover staff shortages during the pandemic.
The warning comes as a national Ofsted report found that heads say they face "unsustainable" and "overwhelming" pressures this term because of the impact of the coronavirus.
Michael Tidd, a primary school headteacher in West Sussex, told Tes that he had seen a nearly fivefold increase in the amount of teaching time he had to cover already this term.
He said: "I am concerned about the difficulties schools are having covering for when teachers need to self-isolate because of possible contact with the Covid case.
“Schools can’t afford to cover the supply costs for the numbers of absences we have this term because teachers are self-isolating.
"In a normal term, I might expect to cover six days.
“This term I am already close to doing 30. Some schools will set aside a supply budget but the cost of this is going through the roof this term and so school leaders have no option but to cover these lessons themselves.”
He said an issue, particularly in primary schools, is that headteachers are the safeguarding lead. They are having to balance that safeguarding duty with classroom work, as well as additional cleaning and the usual responsibilities for "teaching and learning and providing the best offer for all of your pupils", he said.
Nick Brook, the deputy general secretary of the NAHT, said school leaders "found themselves between a rock and a hard place" with staff increasingly needing to stay at home but no extra funding to meet this.
He added: "If schools have already used up their usual budget for supply cover and they have no indication from the Department for Education that they will receive any additional funding for Covid costs then it is not surprising that, when teaching staff are having to self-isolate, then school leaders are deciding to cover these lessons themselves.
"For many, it will be impossible to balance the budget this year whilst incurring a hefty spend on supply teachers to cover staff who are out of class due to Covid."
He said this was another example of why the NAHT wants the government to commit to meeting the additional costs faced by schools as a result of the Covid crisis.
A petition started by the union calling for this has been signed by almost 19,000 people.
Mr Brook added: "Of course, leaders who are filling in for absent staff will be at risk of adversely affecting their own wellbeing and workload, because as we know, the demands of leadership don’t stop when you are in the classroom, especially at the moment."
Mr Tidd warned that the situation was particularly challenging for leaders in schools who needed to address urgent Ofsted findings.
He added: "We are fortunate in that we have had a recent Ofsted and it went well, but if you are in a school that is trying to get out of special measures or requires improvement then that is a burden you can’t afford."
Mr Tidd is head of East Preston Junior School and a Tes columnist.
A Department for Education spokesperson said: "Our guidance outlines a number of options if schools are facing a shortage of teachers, including using staff more flexibly, recruiting both permanent and short term staff via our Teaching Vacancies Service and utilising trainees.
"Local authorities and trusts are also on hand to talk to schools about any concerns over staffing capacity.
"Schools have continued to receive their core funding throughout the pandemic, with this year marking the first year of a three-year increase to core funding – the biggest in a decade."