Wendy Morris, a Cheshire supply teacher, has won pound;730 in compensation from Cheshire county council after being underpaid for 56 days' work.
Her victory in the county court could prompt more claims. There are at least 40 cases in the North-west and dozens more in the South-west and Midlands.
Supply teachers are paid at a rate that reflects their experience and seniority. National guidelines say they should get 1195th of the annual salary they would get as a full-time teacher for a day's work - equivalent to 6.48 hours. Many supply payments equate to fewer hours.
But there are fears some staff may not claim the back pay because they are worried heads will then not give them any more work. Mrs Morris said when she first queried her pay with heads in Cheshire she was no longer offered work there, but she is still doing supply for another authority.
A Wokingham teacher, who did not want to be named, said: "I have dipped a toe in the water with various schools, but it is a complete no-no. They can't afford it. "
Mrs Morris, 56, complained after she was paid for five hours in one school, five-and-a-half in another and six in a third. She only realised she was being underpaid in Cheshire after she started doing supply in another authority.
Her action was backed by the NASUWT union and the ruling should apply to any teacher employed by a council, although not to teachers working through a supply agency. Mrs Morris said: "I hope it improves the situation for other people because it wasn't right. It was a matter of principle. A fair day's pay for a fair day's work. That's all I was asking for."
Cheshire said it still believed supply staff could be paid by the hour - but the case established that a "day" was at least 6.48 hours.
"If a school chooses to pay for fewer than 6.48 hours, it cannot expect the teacher to do a full day's work," the council said.