One of the relatively unique features of Montessori schools is multi-age classrooms. There’s a span of at least three years in each class, and they can go up to six years. (I’ve taught a class of 6-11 year olds, myself.) To be honest, I always think it's ironic when schools go to a two-grade "blend" and it's treated as some sort of daring innovation. Montessorians have been "blending" much wider age ranges for a century, and before that, one-room school houses were the norm. In fact, single grade, age-based classrooms are historically a very new invention.
Usually, we talk about all the benefits of multi-age classrooms for the children, and there are many, but I want to indulge myself for a few moments and consider a few benefits to teachers. (These are really two sides of the same coin: they make for a healthier environment for the children, too. Besides, a happy teacher is a better teacher.)
From my perspective as a guide, the biggest advantage of having children for multiple years is the luxury of time. My impact on a child's learning is limited until we’ve built rapport and the child trusts me enough to willingly follow my lead at times. For some children, that's almost instantaneous. Other children need a long time to feel safe in a new environment and with new adults. For those children, a new class and new teacher every year present a real problem. If it takes six months to relax and feel safe, that only leaves 2-3 months of school each year for happy learning.
When children have several years in the same classroom, we can offer them the time they need. Six months every three years or so isn't a big deal, even if you discount the value of all that emotional learning itself. Moreover, because only part of the class turns over each year, the guide has to build rapport with a relatively small number of children each year. That's so much easier for everyone. I’ve always wondered how teachers in more traditional age-grouped classrooms manage this. I imagine they have some good tools that I've never discovered, but it sure seems like a lot of stressful work.
Another challenge that is greatly simplified (usually) is building a healthy classroom culture. In a multi-age classroom, we don't have to start from scratch every year. The cultural norms already exist, and the returning children are exceedingly effective at communicating those norms to the new children in the class. While that’s not always a good thing—there's no reset button on the classroom culture—on the whole, I find it far easier than trying to build a brand new culture.
These are only two benefits of a muti-age classroom. They just happen to be the ones that are most obviously beneficial to adults as well as children. Why should we care about that? Teachers, at least in the US, are expected to take on an inordinately difficult task. Anything that creates space for teachers to authentically connect with the children in their care will ultimately serve the children.