The Wiki's History section dates the genesis of the practice to 2010:
Eric Mazur developed peer instruction in the 1990s. He found that
computer-aided instruction allowed him to coach instead of lecture. Lage, Platt
and Treglia published the paper "Inverting the Classroom: A Gateway to Creating
an Inclusive Learning Environment" in 2000.
All I can say is this ignores a lot of dedicated people and initiatives dating back to at least 1968, when my Grade 6 "enrichment" class was led by one of the pioneers of inclusive learning, Beverly Muir, currently the Principal at Humberwood Downs Junior Middle Academy in Etobicoke, Ontario, and who a quick internet search shows was honoured by The Learning Partnership as one of the most Outstanding Principals in Canada in 2010.
What was flipped about Muir's classroom in '68? First of all her, desk was at the back. Our desks, rather than being in rows, faced one another in groups of two to four students, as is common now (at least in Toronto). But she went one step further and allowed us to decorate our workspaces, creating "cubicles" with reams of brightly coloured corrugated cardboard, complete with windows that opened and closed or whatever other accoutrements we could think of, so that we would feel comfortable and proprietary of our space and even have some privacy inside the shared classroom.
There was a filing cabinet near the front of the class (far from her desk) that was either full of resources or a place for us to file our work, or both. Today its function would be handled by some kind of classroom management system in the cloud. We had lots of pets, including a class guinea pig, mice (Romeo and Juliet), a frog and a snake (that didn't end well), and others. We went on masses of field trips, the longest to Quebec City, and we had "discussions," where we all sat in a circle and exchanged opinions about some topic we had studied, training us not only to speak up and debate a position but to listen and acknowledge one anothers' worth.
And we did projects. "Roma est une pulchra terra," that's from my study of Latin with my cousin David, also in her class. I'm not even going to look that up to see if it's right. We did a presentation on Parents' Night, which consisted not just of an obligatory visit to the classroom with some of the kids' work on display but an Exhibit Hall in the gym in which we stood beside our work as the parents walked around.
I also directed my first play, completely without adult help, casting my whole class in a script cobbled together from Peanuts cartoons. Muir encouraged us to pursue our own interests, of which mine was cartooning, for instance by getting me Charles Schulz's address so I could write him and ask him questions about the strip.
Some teachers today may worry that flipped classroom means a loss of control, but in my experience there was never any question of who was the ringleader of my Grade 6 circus. Her method was simple: show respect. It was the first time I ever knew a teacher to politely interrupt a conversation with another adult to give ear to the question of an eleven year-old who was quietly waiting to be heard. It's the first year of my school life that is chock full of memories and relationships that extended beyond that year through a shared knowledge that we were involved in something special. This became even more evident once were back in the mainstream.
I've heard the word "transformative" knocked around in descriptions of flipped classrooms. All I can say is, that year changed my life, and those of my peers as well. This is powerful stuff, made more powerful by the advent of computers and the internet into the mix, trifles we didn't know of in the late 60's (we had something called a library and film reels). It demands a ton of the teacher who finds herself suddenly at the helm of a ship rather than the front of a train (pardon my clunky metaphors), and anyone deciding to try it with the idea that it will run itself is bound for a fall.
But if a teacher is willing to trust her students and to really teach, rather than tell, it can offer great dividends both for the teacher and the students for a long time to come.