During my first year of high school, I had a vague idea there was some kind of correlation between humility and good teaching. All of the other characteristics shared by my best teachers made sense: they were knowledgeable, down-to-earth and interesting. Humility always seemed like an unnecessary aspect. It made teachers more palatable, but why should it make them any better at teaching? I can get information just as well from someone who's arrogant. I found out later that my perception was way off. Many of my teachers were, at the very least, well aware of their own intelligence, but they nonetheless had something else in common that felt at first glance like humility.
Seven years later I tutored at a learning center in Cincinnati. I taught history, English and biology, but the vast majority of my time was spent on math. I had a few people taking remedial Algebra 2 in particular. About halfway through the summer, I had an idea: I wanted to learn and then teach some of the mistakes that had been made in the development of algebra. I dumped that idea almost immediately. I didn't have the time, and neither did they. Everyone needed to be finished with the program by the end of summer. Beyond that, the idea sounded pretty stupid when I said it out loud. What would be the point? Nonetheless, I thought it was a good idea. It nagged at me for a while, but I couldn't figure out why.
It turns out the answer to both of those questions is the same answer to why discussion classes are better than lectures: my "humble" teachers - who only seemed humble because they were asking a lot of questions instead of talking at me, a new experience to someone just out of grade school - and my idea for math both entertained the far better philosophy and culture of education. They treated education as an ongoing process for everyone involved, including teachers and professionals. Instead of a top-down approach in which students passively receive information, the idea was that everyone together as a group was working toward figuring something out. As it turns out, that's an obvious lie, but it's a very useful one.
Teachers, especially in high school, have been teaching the same thing for years. They don't need to figure it out (hopefully). Furthermore, they're obviously not peers of students. Classrooms have a very clear power dynamic in which students are subject to a teacher's rules. The only issue is that authority - me telling you something - opposes the best kind of learning - you figuring it out. That's a real problem, because discovery will always be a far more potent learning experience than lecture. And so the best teachers learn to combine the two, to mute the power dynamic as much as reasonably possible and to lead students to figure it out "on their own."
It's an illusion through and through, and not even a very convincing one. Students always know the teacher has an agenda or at least an interpretation. The teacher chooses what students read in the first place. Furthermore, there are numerous instances where teachers have no choice but to take the reins. A student can drag discussions off-topic or even say something downright stupid, which for whatever reason might seem convincing. But the more that can be avoided, the more a class can seem like a group effort, the more educational - and more enjoyable - it will inevitably be. It's only a mild psychological shift, but it ends up having profound effects. Students go from feeling like a subject to feeling like an active participant, and they're not only learning from the teacher but from their equals as well, which makes them more receptive.
Teachers always talk about how they often learn more from their students than their students learn from them, but to be honest, most of the time they aren't learning a whole lot that's new. Many professors I know have been only slightly tweaking the same material for years. But nothing makes a class boring more quickly than a teacher who is himself bored. Even if it is a lie, teachers should make a class seem like a process of discovery as much as reasonably possible. Like many things that seem to be little more than feel-good exercises, creating an atmosphere of "figuring something out (together)" actually has significant practical effects.
