Students of all ages were confused by the wide variation of grades given by different teachers to their class and homework. Even A-level students were left foxed by the red-penned comments on their work.
One 14-year-old told the Bristol University researchers: "Teachers use 1 to 4 for effort, and A to E for attainment. Or it might be the other way around."
Grades were rarely used in primaries and many had a policy of only making positive comments about work. But pupils who received smiley faces, ticks and comments like "good work" wanted more explanation.
One primary pupil said: "'Good' doesn't help very much. He's just saying that it's not really very good."
Pupis of all ages depend on the standards set by teachers when assessing their work, although many refer to past work as a comparison.
Less than a quarter of GCSE and A-level students had opportunities to assess their own work. For those who did, it often consisted of using maths answers at the back of the book to check against. Male students were more confident about grading their work than girls.
Prompt, one-on-one feedback was felt by most children to be the best way to bridge the gap between performance and goals but opportunities for such assessment were limited.
The research, commissioned by the Qualifications and Curriculum Authority, has led to advice to schools to review marking policies and encourage teachers to share their goals and expectations with pupils.
Learning for Assessment site at www.qca.org.uk