The level of support heads get from their school improvement partners (Sips) varies according to whether they are local authority officials or other serving heads, Government-led research shows.
Sips were designed to give all heads a single official to deal with. Representing both local and central government, they are supposed to challenge schools on their performance and organise more support for them when they need it.
But a York Consulting report has found that Sips who also work as full- time local authority officials offer more in the latter role than those who are serving heads.
"Those headteachers allocated with serving headteacher Sips less commonly identify that their Sip has effectively brokered support needs or been a key influence in changing schools' approach to self-evaluation," it says.
"Given the demands of their own schools, serving headteacher Sips are more constrained than other types of Sip in terms of delivering additional support resource for schools they work with."
The finding has big implications for the Government's National Challenge school improvement which will use the Sip approach to help secondaries with low raw GCSE results improve.
National Challenge advisers are to be recruited, partly from the existing pool of Sips, to work in the same way, although they will spend 20 days a year with each of their schools instead of five.
The study found that overall, the Sips that effectively brokered their schools' support needs were in the minority, according to heads. The figure dropped as low as 14 per cent for primary heads who had serving heads as Sips.
But in some areas, local authorities continued to provide the support role. The Sip programme had helped develop a more accountable school culture and a more consistent focus on improving outcomes.