Examination results for every secondary school in Wales were published nationally this week - for the first time since performance tables were abolished in 2001.
An Assembly government spokesman said the decision to release the statistics reflected its "continuing commitment to open government and wanting to ensure proper and accurate use of the data".
But unions are concerned that the information may be used to construct league tables of schools, based on raw exams results.
Brian Rowlands, secretary of the Secondary Heads Association Cymru, said:
"Parents must have information, but we wouldn't like it to be produced as league tables.
"We feel not having tables has allowed people to concentrate on teaching rather than results - they are more concerned with teaching and learning than league tables."
But David Reynolds, professor of education at Plymouth university, who lives in Wales, believes data will help schools to improve.
However, the statistics need to include value-added measures - as in England - so readers can tell whether a school's results "are truly good or just reflect their catchment area", he added.
The Welsh statistics detail each school's GCSE and A-level results for the past five years, giving figures for five A*-C grade GCSE passes, five A*-G, one A*-G, and the core subject indicator of EnglishWelsh, maths or science at A*-C.
They also spell out the proportion of pupils entitled to free school meals, and the percentage of half-day sessions missed due to absences. The timing of the Welsh stats release followed a request from TES Cymru under the Freedom of Information Act.
Secondary results 4 Key statistical indicators at school level 2000-5, www.wales.gov.uk