Hertfordshire's teachers miss more than 1,000 days a week due to illness, which education experts say supply teacher agencies exploit to "cream" money away from schools.
The National Education Union called on the Government to do more to stop supply agencies charging large fees to schools already struggling financially.
The Department for Education estimates this costs schools up to £75 million a year nationally.
In the last academic year, Hertfordshire schools lost 40,904 days to teacher sickness, according to the latest DfE figures. This equates to 1,049days a week over the school year.
If schools in Hertfordshire covered every sick day with a supply teacher, on the average daily rate of £124, they would be paying £5.1 million year.
And this does not include the “substantial” and undisclosed fees which agencies charge schools. The National Union of Teachers claimed some agencies charged up to £100 per day.
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% of supply teachers now get work through agencies nationally, as opposed to 50% in 2010.
Supply teachers are also asked to fill temporary positions at schools, for a contract period of between one and three terms.
In Hertfordshire, in 2017-18, 13% of schools were reporting a staff vacancy, and there were 93 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, the Education Secretary, Damian Hinds, 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."