History lesson
When the 3 tier system was introduced it was in an attempt to provide schooling that matched the physical, social, emotional and academic development of the children.
The Plowden report on primary education, published in 1967, concluded that there was a distance "middle phase" to children's learning between the ages of eight or nine and 12 or 13, and that 11 was not the most appropriate age for transfer to secondary school.
Broadly speaking, children were 'socialised' in smaller nurseries and 4+ units and transferred to smaller Lower Schools. At aged 9 when children generally became more socially adept and confident a transfer was affected to Middle School.
The principal of the Middle School was to take children from childhood to the cusp of young adulthood in a smaller community at aged 13.
The next transfer would effectively take children through adolescence toward full adulthood.
The 3 tier system was built around the developmental needs of the child and the nurturing of personhood.
The shift away from 3 tier has been driven entirely by the need to compartmentalise children according to National Tests and not the needs of your children. Schools are more 'community' than 'test conveyor belt'. Middle Schools are more about growth and development than test results.