I went to a Montessori school from kindergarten through 8th grade. Before 6th or 5th grade we never had "real" homework. around 6th grade they started introducing more structured and rigorous homework, and by 7th and 8th grade we had homework that very closely resembled the type of homework we'd be doing in high school (and as I discovered in high school, was more challenging sometimes). Throughout all that time we never had desks. The majority of our time was unstructured time with which to accomplish whatever tasks we'd been given for the day. We could sit where we pleased as long as we were behaving appropriately. At various times during the day one of the teachers (there were two) would take a small group of kids aside for a lesson. I loved nearly every day of school there and even now people who graduated 8th from there 20 years ago still come to alumni reunions there, it had that great of an effect on them.
I did take the power of hindsight, but I realize now how good my elementary education was at that school.
And how many kids don't get to have that same level of education. It wasn't cheap, but it's a shame all kids don't get to experience that.
The most important thing I learned there was how to think critically and come to my own conclusions on matters.
EDIT: Now that I'm reminiscing, my favorite memory is over the course of a few afternoons in 7th grade retreating to the comfy reading area and picking up and reading the classroom's copies of Catcher in the Rye and Lord of the Flies.