The Joint Council for Qualifications, which represents exam boards, wrote to schools and exam centres this week, clarifying this year's regulations.
Dr James Sinclair, director of the council, said that teachers would be allowed to check exam papers and pupils' identities, then leave, rather than waiting for an hour, the time limit for late pupils.
John Dunford, general secretary of the Association of School and College Leaders, said good sense had prevailed and that invigilation would be done by specialists rather than reluctant teachers.
The workload agreement requires schools to hire support staff to oversee exams.
Schools have implemented this in different ways: West Park school in Derby paid its lunchtime supervisors to oversee exams, while nearby St Benedict RC school hired parents and former pupils.
Chris Keates, general secretary of the NASUWT, said the union would have sued for breach of contract if exam boards had tried to force teachers to invigilate. "I am glad that they moved quickly in the end," she said.