When a supply agency signs up with a school, they must obviously explain how their fee structure works. In my mind the most likely possibility is: 1) A flat fee added applied to any normal daily supply. Surely the agency doesn't just say to the school, "Sorry I'm not going to tell you how our fees are calculated." So with that being the case a school must surely be able to in theory figure out roughly what a supply teacher on daily rate earns? Do you think supply agencies who operate an umbrella or PAYE system explain to the schools that they operate this way? In particular with umbrella set ups and long term bookings, this might cause schools to think that candidates were being greedy in their demands, when in reality they were completely unaware that the candidate was having to foot the bill for both employer and employee NI?