A spokesman for the SQA said some 400 centres had been invited to take part in spot checks. Only half the schools responded.
Under the derived grades system, teachers estimate the grade they expect pupils to achieve in end-of-year external exams. If they underperform, they are automatically upgraded as long as enough other pupils at the school achieve their predicted marks.
However, the system has been criticised for offering an advantage to large, high-attainment schools. Small schools or departments presenting small numbers of candidates were said to be at a disadvantage.