School-leavers' grades in Sweden are being decided by teachers lacking training and support, according to the National Agency for Education. Not enough is being done to ensure students are graded fairly, it said.
Anna Barklund of the NAE said guidelines on grading should be included in teacher-training courses, although many teachers are not trained at all.
"Sweden needs more qualified teachers," said Ms Barklund.
Class teachers have sole responsibility for grades in coursework and exams, but staff are uneasy about bearing the responsibility without adequate support.
Ernest Bolster, who teaches French and English to 16 to 19-year-olds in Odenplan, said: "The criteria are too vague. We have no help from external examiners, as is the case in other countries."
Those who do teacher-training get little instruction on grading. During his teaching practice, Mr Bolster said he relied heavily on other teachers to get a sense of how the grades work.
The report found that in upper-secondary state high schools, only 79 per cent of teachers are qualified. But in their independent equivalents, the figure is 51 per cent. And figures vary widely across subjects: 68 per cent of maths and Swedish teachers are fully qualified, compared with 38 per cent of Spanish teachers.
Tomas Johansson, chairman of the Association of Independent Schools, said the figures are misleading because many independent schools employ non-teacher professionals in subjects such as business studies and IT.
The NAE said problems of teacher recruitment had obliged principals to hire teacher candidates as substitutes or part-timers.
Despite the criticism, Swedish 15-year-olds performed well in international assessments. Only three countries have better rates of literacy.