Teacher grade forecasts are 'too optimistic'
Teachers are often too optimistic when predicting pupils' grades, suggests a new analysis of exam board data.
In future, these forecasts will be crucial to universities when offering places to pupils.
That is because A-levels results in in England will depend solely on exams taken at the end of two years' study.
The forecasts "will be the only thing universities will have to go on", said Cherry Ridgway of the Association of School and College Leaders (ASCL).
But the accuracy of teachers' forecasts is falling, the study suggests.
The researchers compared GCSE and A-level results from the OCR board with predictions sent in by teachers just before the exams.
At A-level some 43% were correct in 2014, down from 48% in 2012
For GCSEs, 44% of predictions were accurate in 2014, compared with nearly 47% in 2013.
Researchers Tim Gill and Dr Tom Benton of Cambridge Assessment suggested that part of the reason could be the government's removal of unit exams taken in January, which had made it harder for teachers to predict pupils' final results.
Currently universities use AS-level results when making offers - but from next year they will no longer count towards the final A-level grade.
Research by ASCL suggests 70% of schools will continue to offer AS-levels next year but there are concerns that the results will no longer be an accurate predictor of final A-level results.
"New exams and new content, as yet unseen, mean it's going to take a little time for teachers to be accurately able to predict results," warned Ms Ridgway.
Russell Hobby, general secretary of the National Association of Head Teachers said predicting grades was extremely difficult to do with any certainty.
"This means that, should the accuracy level continue to fall, students will face more uncertainty as they wait for their results, which is not the reward they deserve for their hard work," he said.
However teachers' predictions were usually not far out, the figures show - in 2012, 92% were either accurate or within one grade - but this fell to 88% in 2014.
Teachers are more likely to be over-optimistic in their forecasts, with 43% predicting higher A-level grades than pupils eventually achieved in 2014.
For GCSEs, teachers predicted higher grades than pupils actually achieved in 42% of cases.
For both types of exam only 14% underestimated the final grade.
The researchers found that independent and grammar schools had the best track record for accurate forecasts at A-level.
But they suggest this might be because students at these schools generally achieve higher grades - and higher grades are easier to predict.
The OCR board said the figures showed it was crucial for more teachers to become involved in examining.
"It underlines the need for more teachers to become examiners so they can really understand the nuts and bolts of how their students can perform", said Sylke Scheiner, the board's director of assessment standards.