Pupils could be unfairly graded in GCSEs and A-levels if they miss exams due to illness, a school said this week.
Staff at Tamarside community college, in Plymouth, complained after GPs refused to issue sick notes to satisfy exam boards.
Board rules say that if a student misses an exam and the school or college cannot be sure they were ill, they should get a doctor to certify the sickness.
But a surgery near the school has stopped issuing sick notes on the advice of its local medical committee.
The British Medical Association also says doctors can refuse to write the notes, and one senior Derbyshire GP said he would not write them.
In a newsletter sent to surgeries, Devon medical committee said: "This is an absolute no. This is not part of your NHS contract."
The committee said that if a school insisted on a sick note doctors should charge the parent pound;13.
An earlier newsletter had warned doctors that pupils had incentives to miss exams because if they went sick they could be awarded a pass based on their projected grades.
Keith Ballance, head of Tamarside, said the guidance was "an absolute disgrace".
"The whole thing stinks. We serve a poor community. If you are a parent and you face a choice of paying pound;13 or providing a family meal, that's an impossible decision to make."
Debbie Galbraith, the committee's executive officer said it was not doctors' job to "police" exams.
The Joint Council for Qualifications said medical information was required to protect the integrity of grades.