My daughter is also a 3rd year primary (kindergarten) student in an AMI Montessori school (AMI being the 'strictest' of the accredation programs).
Kids do work together a good bit, particularly as they get older (5+). I just went on the Lower Elementary tour (1st-3rd grades) since my daughter will move up next year and they stressed how most of the work at that age level is in groups as kids are social beings at that age and just love being together and sharing ideas. When I visited, the 1st graders were working on spelling with their teachers, 3rd grade girls were doing a science project, the 2nd and 3rd grade boys were working on math (in pairs, but all were doing math).
In my daughter's current class, I have yet to see her alone this year. She is constantly surrounded by the younger kids as she loves to read them books and even at just 3yo they are a very patient audience. In the afternoon, only the 3rd years stay and there are just 4 kids at the moment (they move up according to birthday throughout the year, so there will be 9 of them by January). She does do some work alone during that period, but only because at this point she's ahead of the other 3 kids. That is in part because she's the oldest in the class, but also because she moved up to "kindergarten" in January (late February birthday) so she's had a good 5 months other the other kids.
As far as kids not being interested in a topic, avoiding it, therefore not learning it/doing well, that simply isn't allowed. The teacher works closely with each student daily and can see what areas a child is avoiding very quickly and will then work one-on-one with them in that area and pair them up with an older student. The Montessori mantra, so to speak, is see one, do one, teach one (interestingly, also the method used in medical school and medical residencies so my husband was pretty impressed to see them apply the same theory to him at 25yo that our kids are doing as preschoolers). So not only do kids ultimately have a great understanding of the materials, but there's also lots of interaction among peers.
Like the PP said, my daughter has learned an incredible amount so far. She is independently reading chapter books, doing addition, subtraction, and beginning multiplication (all single-digit). She can name and locate all the continents on the map/globe, as well as many countries on each continent. She's been working on the map of Africa for ahwile and knows the names and locations of many more countries than I do! (This work is done mostly through puzzles, so it's not just staring at a map and memorizing, it's actually fun.) In science she's made books about animals, labeling the parts of their body. She's also writing stories (in cursive) and illustrating. I truly can't say enough about how great the education has been so far.
And FWIW, some kids leave her school after K and either go to public school for 1st grade or another private school (typically a K-12 private school, just as it is easier to get in in K or 1 and then have the security of not needing to apply to schools at the middle or high school level-our school goes to 6th grade). The kids who have move out to public and private have been extremely advanced for their grade level and bored for the 1st year or so of school. 2 of my daughters friends went to Sidwell Friends (which I mention only because most people have heard of it due to the president's kids going there) for K and 1st grade and were very bored as they already knew all the work presented. By 2nd grade I am sure it evens out, but in the meantime these kids were reading books while their class was being taught to sound out words. In math they were learning counting while the Montessori kids could already do basic multiplication. It wasn't necessarily a bad thing as it gave them lots of time to socialize which is also very important at that age, but they did actually complain to their parents that they were bored and wanted to go back to their old school where they 'learned'.