Testing all secondary pupils for coronavirus upon their return to the classroom is a "logistical nightmare", teachers have said.
Students will take a series of three Covid tests from 8 March.
Jo Tunnicliffe, head at North Kesteven Academy, in Lincolnshire, said it was impossible to test 1,000 pupils at the same time.
The government said schools "had the flexibility to consider how best to deliver testing on a phased basis".
Department of Education guidance stipulates that pupils will be asked to carry out home tests twice a week after they have been tested three times at school within the first two weeks of term.
Ms Tunnicliffe said: "Those first three tests which have to be three to five days apart each, it's a logistical nightmare."
Although rapid Covid testing for key worker children has been in place at the school near Lincoln since January, the most tests carried out in one hour has been 25 - equivalent to 125 in a school day.
Ms Tunnicliffe said: "So to look at that across 1,000 people, that's going to take some time to get through.
"We're looking at how we upscale that to get everyone through and into the classroom as soon as we can."
Because school staff are required to carry out the tests, resources have to be pulled from elsewhere which was "not ideal", she said.
Her concerns have been echoed by The National Education Union, which said the issue of mass testing would be "immensely challenging".
Dr Mary Bousted, Joint General Secretary, said: "Inevitably this huge task, which also includes dealing with issues of consent, will put pressure on scarce resources at a time when schools will be struggling to adapt to full opening."
Jane Girt, head teacher at Carlton Bolling School in Bradford, said a phased return was likely because 600 children a day would need to be tested.
"It's the scale of it. A school like ours - 1,500 students, 250 staff - and we've got to test them each in a week, it's the logistics of that that we really need to get our heads round."
She said they would have to set up a second test site as well as decide upon which members of staff could do testing because of the need to deliver face-to-face teaching.
Pepe Di'lasio, head teacher at Wales High School in Rotherham, said there was a desperation for "some kind of normality" but some staff were "daunted" by the return.
"I think many of us would have liked to have had the vaccine as part of the next step and I think what we're really wanting is when students come back, that there's not an increase in transmissions so we have got to make sure all the protocols are in place."
The Department for Education said if schools had followed its guidance around staffing capacity but still had concerns about their ability to administer tests, they should talk to their "local authority or trust point of contact".