It also has significant social advantages. Younger ones tend to be "mothered" by the older ones who develop a sense of responsibility. All children have the opportunity to develop a wider range of friendships, which contributes to a friendly atmosphere in the school.
We have had only one problem when my eldest child, who is also one of the eldest in her year group, found herself, after two years "at the top" in nursery and reception, in the middle of a Year 1 and 2 class. She did not like it, but had to learn to cope with it. This is an experience that the oldest and youngest in the year will not get in single-year classes.
The system had an another advantage this year when the school was over- subscribed. With the help of additional funding an extra mixed-year class was added to the school when a single-year class would not have been economic.
It has settled in with minimum disruption because it fitted in with the school's existing philosophy.