Schools grouped in chains of three or more produce better results than standalone academies, new research has found.
A report, published Thursday by the National College for School Leadership, found that, in the three years to 2011, the proportion of children achieving five good GCSE passes increased by 15% among schools in larger chains.
The same measure improved by only 12.2% in standalone schools, and 11% in chains of two. Larger chains also had a higher proportion of schools rated outstanding by Ofsted.
The researchers also found that chains that had a layer of "executive leadership" to tightly manage its headteachers, performed better than more informal collaborations.
Policy analyst Robert Hill, who led the research, credited chains' success to factors including recruiting high quality staff; a strong emphasis on leadership development; intensive use of performance data; and a defined model of school improvement.
Chains “are growing an able new generation of school leaders and enabling school leaders to spend more time on their core business of teaching and learning," he said.
But he added that chains were "not a panacea", and stressed that, "The performance of weaker chains needs addressing".
The report also found that:
• As of 1 January, there were approximately 600 schools in chains. These include 93 of the coalition's 'converter academies’, which had either joined up to share their strengths with weaker schools, or had been forcibly handed over to a chain after under-performing.
• Only two chains take more than 6% of their schools funding to run their central services. Three-quarters top-slice less than 4.5%, while half take less than 3%.
• Nine chains have 10 or more schools. The largest is the Academies Enterprise Trust (AET), which currently has 27 schools, and expects to have 33 from next year. E-Act and the United Learning Trust also run more than 20 schools each.
• The rapid growth of chains is down to factors including the need to find cost efficiencies, and a desire to extend leaders' expertise to more schools.
• The majority of chains – 22 out of 28 surveyed – said it was important that schools be located in geographical clusters, to enable them to work together.