Schools are finding it difficult to compare how they are doing with similar ones elsewhere because tables of test and exam results are no longer published in Wales.
The implication is that the lack of information is making it harder for schools to set realistic targets for improvement.
The comments, in inspection agency Estyn's annual report, come after England's chief inspector criticised the abolition of tests and league tables in Wales last year. David Bell said Welsh parents were being "let down" by the lack of information available about the education service.
Secondary performance tables were abandoned in Wales in 2002 and were never published for primary schools. The Assembly government was concerned they were misleading, incomplete, and could be divisive and demoralising.
A TES Cymru poll last May showed two-thirds of parents wanted league tables, but only 10 per cent asked for a school's results when considering it for their child.
In England, the Office for Standards in Education issues schools with an "autumn package" of data, including performance and assessment reports (PANDAs), which compare a school's test and exam results to national averages and those of similar schools (based on free school meals and pupils' previous attainment).
Schools in Wales will not be able to access such comparative data until later this year, when a national pupil database launches.
Susan Lewis, chief inspector of education and training in Wales, declined to comment further on her annual report, published this week, saying the decision to end performance tables was a matter for the Assembly government.
But, in a section of her annual review dealing with school target-setting, she notes: "Local education authorities do not share information with each other enough and schools find it difficult to compare their results with those of similar schools in other LEAs."
Overall, though, schools in Wales have got better at setting themselves targets and measuring their improvement. LEAs also help by analysing schools' examination results and discussing pupil achievement with school managers. And this year more heads have agreed to share their pupils'
results with other schools in their LEA, allowing more and better informed comparisons.
But, notes the report: "Often, LEAs do not check whether schools use the information fully."
An Assembly government spokesperson said: "LEAs have information about their schools, and it is part of their job to help measure performance and set targets. The chief inspector's report confirms that this is happening.
"LEAS have a much deeper understanding of the strengths and weaknesses of their schools than could be derived solely from bald statistics."