Lincolnshire's teachers miss more than 1000 days a week due to illness and the National Education Union (NEU) has accused supply teacher agencies exploiting this to 'cream' money away from schools.
And the union has called on the Government to do more to stop supply agencies charging large fees to schools already struggling financially.
The Department for Education (DfE) estimates this costs schools up to £75 million a year nationally.
In the last academic year, Lincolnshire schools lost 32,157 days to teacher sickness, according to the latest DfE figures, which equates to 825 days a week over the school year.
If schools covered every sick day with a supply teacher, on the average daily rate of £124, they would be paying £4 million a year.
Andrew Morris, assistant general secretary of the NEU, said: "Supply agencies cream off millions of pounds every year from schools, charging them substantial fees while paying supply teachers appallingly.
"The DfE is actively supporting agencies when it could be adopting a Northern Ireland model, where a Government-backed scheme puts schools and supply teachers in direct contact, saving schools money and paying teachers more."
According to a NEU survey, 81 per cent of supply teachers now get work through agencies nationally, as opposed to 50 per cent in 2010.
Supply teachers are also asked to fill temporary positions at schools, for a contract period of between one and three terms.
In Lincolnshshire, in 2017-18, seven per cent of schools were reporting a staff vacancy, and there were 18 posts filled temporarily.
However, if a school wants to make a temporary teacher permanent, they will have to pay a finder’s fee to the agency.
Last summer, Damian Hinds MP, Education Secretary, launched a website to link schools directly with supply teachers.
He said: “Every pound that’s spent on excessive agency fees, or on advertising jobs, is a pound that I want to help schools spend on what really matters.”
Schools are also using AirBnB-style apps to dodge expensive agency add-ons.
Slava Kremerman, co-founder of supply teacher app Zen Educate, explained: "When I speak to headteachers, they tell me about the long mornings spent on the phone trying to find teacher cover, only to be slapped with massive fees at the end of the day."
He says apps and websites mean "less time spent on the phone, and more money to spend on books, sport equipment and computers".
A DfE spokeswoman commented: "We have launched a national deal to support schools with getting value for money when hiring agency supply teachers and other temporary staff.
"The deal includes a list of preferred suppliers who are open about the rates they charge, and also help schools to avoid finders fees."