Money invested in early years and 14-19 initiatives will be "thrown away" unless major reforms are made to middle-years schooling, according to an expert advising the Assembly government.
Professor David Egan, who chaired an academic review of 8-14 teaching last year, said the so-called "bit in the middle" must be urgently addressed by policymakers.
Speaking at the Institute of Welsh Affairs' Learning Pathways conference in Cardiff last week, Professor Egan said that 8-14 must become a distinct phase of education in Wales.
"Some form of systemic reform is required," he said. "It's unlikely that the objectives of the foundation phase and 14-19 learning pathways can be fully achieved unless changes are made to the 8-14 experience. There's a significant financial concern: we would be throwing money away unless we make changes."
While most pupils make good progress throughout the 8-14 period of their education, a significant minority become disengaged. Research suggests that 40 per cent of pupils in Wales perform worse in the core subjects at age 14 than they did at 11.
Professor Egan, who is based at the University of Wales Institute, Cardiff, said disengagement begins in the later primary years and accelerates at the start of secondary, and is often associated with basic skills weaknesses, particularly in literacy.
Outside schools, these problems are intensified by low aspirations and negative peer pressure, he said.
While the Assembly government has ploughed money into initiatives such as the play-led foundation phase for three- to seven-year-olds, the vocationally led 14-19 learning pathways and the skills-led Welsh Baccalaureate in recent years, there is a widespread feeling that 8-14 has become a "forgotten phase".
Professor Egan said its reform should become an integral feature of the attainment-raising School Effectiveness Framework, which is currently under development.
He said reforms should focus on improving the quality of teaching and leadership and suggested that Estyn, the Welsh inspectorate, carry out 8- 14 area reviews instead of focusing on individual primaries and secondaries. He has also called for teachers to be specifically trained to work in the phase.
Jane Hutt, former education minister, accepted all the report's recommendations last year, but Professor Egan said he and his fellow academics were waiting to hear from Leighton Andrews, her replacement, before deciding how to continue their work.
A spokesman for the Assembly government said: "The minster is keenly aware of the importance of this phase of education. In particular, he wishes to see major improvements in boys' literacy and is currently considering his whole portfolio before announcing decisions on his priorities and the way forward."
The teaching profession has mixed views about the aims of the 8-14 review and the prospect of further reform. While no one disagrees that there is a significant problem that needs to be addressed, there is little appetite for a curriculum overhaul.
Anna Brychan, director of teaching union NAHT Cymru, said school leaders had long suspected that pupils were becoming disengaged in the early secondary years, and would welcome the focus.
But Elaine Edwards, general secretary of Welsh-medium teaching union UCAC, said schools needed continuity, not more government policy. "What we don't want is another new curriculum or to go back to middle schools," she said.