Before we started homeschooling, DD was at a very small private school -- <50 kids for K-6. The classes were mixed age. It had some very nice advantages. All the teachers (all 4 or 5 of them) knew all the kids; kids were more ability-grouped than age-grouped; kids were able to work ahead at their own pace, particularly in math; the whole school would play together at recess, so that kids of different ages learned to intermix very comfortably. Another advantage was that the school was able to offer a LOT for a very reasonable price, because they didn't have a lot of overhead.
But one thing I discovered this Fall is that even with class sizes as small as 9, it is still difficult for teachers to be totally on top of keeping each kid working at their own level. The K-1st teacher was masterful at this, so much so that she made it look easy and I didn't appreciate how awesome she was.
DD's new teacher this year was not nearly as good at it. Her solution was to start everyone off in the math book, and have them work through every single page at their own pace. Those who were willing to buckle down and work quickly got ahead; those who didn't stayed behind, and (in my DD's case) endlessly worked the same kind of problem they had already mastered.
I also learned, as I got to know parents of older students a little better, that in the older grades pretty much no actual teaching of math happens. The kids are set to work on their workbooks, read the explanations for themselves, and raise their hands for help if they don't understand. The teacher spends the period going around answering individual questions. Kids sometimes sit for 10 or 15 minutes doing nothing while they wait their turn. This is the school's trade-off for individualizing student progress.
Another thing I became aware of (again, when DD moved out of the sweet little K-1st room) is that, because the space for the whole school is small, crowd-control is a huge issue, so that students can concentrate on their work. This means that there is a lot less noisy, active, participatory learning than I thought when I first visited the school. There is a lot of being quiet and working at tables.
These were some of the issues (but only some) that tipped me over to homeschooling.