Exam results and teacher assessments for pupils at every primary and secondary school in Wales have been quietly published online and centrally for the first time since performance tables were abolished in 2001.
The information was released at the end of last month via the national pupil database in Wales, which is managed by the Local Government Data Unit and funded by the Assembly government.
Headteachers said they hoped the information would be used responsibly and not to compile local "league" tables of schools.
But academics said more data was needed to give a full and fair picture of individual schools' performance. Unlike in England, the Welsh school summary reports do not include value-added measures of performance, which take into account pupils' previous attainment and levels of deprivation.
However, the Assembly government has awarded a three-year pound;28,000 contract to the Fischer Family Trust to develop value-added measures in Wales.
For the past two years, schools and local education authorities have received pilot data on how well their pupils have done, taking into account previous attainment, deprivation, date of birth, gender and other factors.
David Reynolds, professor of education at Plymouth university but based in Wales, said: "The data being in the public domain makes a lot of sense since it was possible for parents and others to get it in any case through the Freedom of Information Act.
"The upside is that parents will be able to make more informed choices. The downside is that without the value-added data, we do not know whether good performance is because of a good school or good home backgrounds.
"The other downside is that local and national newspapers will be able to compile the hated league tables that we set our face against in Wales. With the imminent arrival of Welsh value-added tables, though, I think that the benefits outweigh the losses."
Gareth Jones, secretary of the Association of School and College Leaders Cymru, said he had no problem with publication of performance data on individual schools, providing it is used responsibly and does not lead to competition between schools.
"To provide better education in the future, we need to collaborate. And competition kills collaboration," he added.
Jane Davidson, education, lifelong learning and skills minister, scrapped secondary performance tables in 2001 (primary ones were never published in Wales), following a public consultation.
At the time, she said tables couuld be divisive and demoralising to staff and pupils, encourage schools to focus on exam results at the expense of a wider curriculum, and encourage unfair and simplistic comparisons between schools.
An Assembly government spokesperson said its policy on performance tables is unchanged.
"Publishing performance information that can be ranked in the form of 'league tables' is not the best way of presenting this information to schools and parents, and the wider public," he said.
"However, to ensure that data in the public domain is more accessible, while protecting it from misuse, performance data for secondary schools was made available via the Assembly government's website in November 2005.
"This year we have moved the facility to the national pupil database, and extended the coverage to primary schools."