We are really a modest lot
Self-praise does not always come easily to teachers when assessing their own performance, according to inspection body Estyn. Inspectors say many schools are achieving more than they believe and teachers are reluctant to celebrate their success.
But Iwan Guy, acting director of the National Association of Headteachers Cymru, described teachers as humble beings who did not like "blowing their own trumpet".
Estyn's comments come after an analysis of self-evaluation, a new strand of the inspection framework introduced last year. Self-evaluation is intended to boost schools' confidence and identify areas for improvement.
Parents, staff and governors fill in questionnaires about standards at their school. But there is still confusion over how to go about it.
The head of a school found to be under-estimating itself said it was due in part to a perfectionist streak among a young staff aiming for high standards. Porth infants' in Rhondda Cynon Taf was one of dozens of schools taken to task by inspectors for not rating itself highly enough.
Acting head Pam Morgan said: "We are always looking at ways of doing things better.
"Our emphasis is on developing good relationships and emotional development, and we knew that was working, but we're a young staff and perhaps we do need to recognise our strengths more."
The school received a glowing report from Estyn earlier this year, but teachers marked down its achievement in three key areas of teaching and leadership.
Inspectors praised the school for its "family atmosphere" and awarded a grade 1 - the highest score - for effective leadership, support of pupils, and the quality of teaching. The school gave itself a grade 2 for each of the criteria.
Inspectors wrote: "The school has under-estimated the impact of its current strategies to improve pupils' achievement and the effect of the positive ethos of the school on pupils' learning."
Mr Guy said: "The profession does tend to under-estimate itself but self-evaluation will be a valuable way for schools to give themselves an honest appraisal between inspections and compare that with Estyn's findings."
Estyn also inspected Glan Afan comprehensive school in Neath Port Talbot earlier this year - a school that had slightly over-estimated itself.
It was awarded top marks for supporting and guiding pupils, but needed to improve its evaluation and standards.
Glan Afan's achievements had slightly over-estimated its achievements and Estyn said that the system of evaluating progress was not yet "sufficiently rigorous".
Mrs Sue Handley, school head, said: "We are now aware of the evidence needed for self-assessment and are more confident."
Rhys Williams, of the National Union of Teachers Cymru, said it would take time for teachers to become confident about self-assessment.