Classes can be organised vertically (5-12 years old), horizontally (16 years old) or a combination (one of 8 to 10 years old and another of 11 to 13 years old).
The most practical is the combination model, with each class having a narrow age range of students at a similar stage of development.
Combination classes are more socially authentic, permit optimum class sizes and are more efficiently staffed.
It is recommended that students between 14 and 18 years old remain in horizontal groupings.
A link below leads to a comparative table of class organisation for an elementary or primary school.
It demonstrates how, for the same number of teachers, a mix of single-age and mixed-age classes keeps the class sizes of older students at least 18 percent smaller than if there were only single-age classes.
The other link leads to an article on the background to multi-age groupings.