Innovations needed to improve the curriculum for pupils in key stage 3 are being sidelined as schools focus attention on post-14 education, inspectors have warned.
Secondary schools have given priority to 14-19 policies, such as Learning Pathways - which offers more vocational options - meaning that children lower down the school have suffered, according to Estyn.
The revised school curriculum introduced by the Assembly government in 2008 gave schools more control over what they could teach in key stage 3. Subject content was reduced to allow schools more flexibility to implement the curriculum to suit their own circumstances.
Estyn's report found that pupils aged 11-14 who attend innovative schools are more motivated and their teachers are more enthused.
"Pupils' enjoyment of their work increases, their attitudes improve, and they develop a greater confidence and independence in lessons," the report said. "Teachers are re-enthused, their teaching skills are broadened, and subject barriers are broken down."
Successful innovation depends on strong leadership from teachers and senior staff, with good support from local authorities, it added.
There was also evidence that schools that have introduced a theme-based curriculum have seen an improvement in pupils' general skills.
However, the report also said: "The impact of the revised curriculum has been reduced in secondary schools because of the priority given to 14-19 developments. Schools have to plan the KS3 curriculum carefully in order to link with the 14-19 curriculum."
David Egan, professor of education at the University of Wales Institute, Cardiff, said: "My feeling is that schools are trying to achieve a lot while facing funding and workload issues, and this shows that we have got to get better at doing things and completing things."
Professor Egan, who chaired a review of eight-14 education last year, said it was "encouraging" that a significant number of secondaries were being innovative, but added: "Curriculum changes of this type have a role to play in improving standards, but the pedagogy of teachers, particularly in developing the literacy of students, is a far more important area for development."
The report said schools should share their best curriculum and teaching practices and provide more training opportunities for their teachers to become more innovative.
The Assembly government welcomed the report's findings. A spokeswoman said: "It provides a valuable early overview of the innovation and flexibility in schools which has resulted from the revised school curriculum which we introduced in September 2008."
Original paper headline: Inspectors: KS3 progress cast aside in favour of 14-19 focus