The testing fiasco has hit almost every school in the UK with up to 25,000 teachers in England already forced to stay at home and self-isolate.
Leaders of teaching unions warned of 'lockdown by default' yesterday as thousands of pupils are being sent home from lessons after showing symptoms of the virus.
The government's prioritisation of NHS staff for tests means that pupils and teachers have been unable to get checked for the virus and have instead been forced out of the classroom.
Some schools have reported up to a fifth of their staff having to stay home, while unions warned the crisis was spinning out of control.
Headteachers have now called on the government to prioritise the education sector for tests as the crisis may make 'staffing unsustainable'.
It was reported yesterday that at least 740 schools have shut or sent children home because of suspected outbreaks, with some sending whole year groups away.
However, some private schools are paying £120 to test symptomatic students and teachers.
One Guildford private school teacher told the Guardian: 'If schools can afford to take the hit, private companies don't have any problems providing tests.
On the other hand, state schools are fast using up the 10 testing kits supplied by government at the start of term – regardless of the school's size.
Thousands of pupils are being sent home from school, sparking union fears of a 'lockdown by default'
Headteachers representing more than 16,000 pupils in Gateshead in north-east England, have written a letter to MPs warning about the effects of the testing crisis.
Gateshead is on the government's watchlist because of its high infection rate and school leaders have now warned that the lack of testing capacity would 'break' some schools.
The letter from the Gateshead Association of Primary Head Teachers, which represents 67 schools, cited 'significant problems' caused by a lack of testing.
Mustafaa Malik, chair of the association and head of Harlow Green primary school in Gateshead, said a fifth of his teaching staff were isolating as well as a 'double figures' number of its 400 pupils.
He said: 'It's just unsustainable. The parents can feel the anxiety growing. It's taken a lot of convincing them that it's safe but we're starting to get a buildup of fear in some communities.'
Today the Government will try to wrest control of the crisis by publishing a 'priority list' spelling out which groups should be