> It would be more helpful to ditch the 'myth' that age-segregated classrooms are an improvement over the one-room school house.

They still mix ages in Montessori schools. There is a stronger self-learning culture, including peer-to-peer collaborations and older children playing a mentorship role to younger ones. The teachers observe, faciliate and are there whenever they are needed.

It's not always so rosy in real life though. One school I volunteered at actually had to do more age-segregated classes (rather than lump Lower Primary together) because it was way too disruptive to mix 6 - 9 years old together. To add context, since the school was specifically for disadvantaged children, most (especially refugees) either join much later or stop suddenly, so couldn't fully benefit from the Montessori system.