Thinking this over, it seems to me that while school as we know it probably does not encourage creativity, creativity in itself is not particularly interesting or useful. Somebody who imagines wonderful stories, but lacks the staying power or literary background to turn those stories into books, will never be a writer. Somebody who imagines a clever new gizmo, but lacks the wherewithal to build and market a prototype, will never be known as an inventor.
This shouldn't be too surprising: Would you really want a little Picasso in your class? How about a baby Gertrude Stein? Or a teenage Eminem? The point is that the classroom isn't designed for impulsive expression - that's called talking out of turn. Instead, it's all about obeying group dynamics and exerting focused attention. Those are important life skills, of course, but decades of psychological research suggest that such skills have little to do with creativity.
Look, for instance, at daydreaming. It's hard to imagine a cognitive process that's less suitable for the classroom, which is why I was always castigated for staring out the window instead of looking at the blackboard. In a culture obsessed with efficiency, daydreaming is derided as a lazy habit or a lack of discipline, the kind of thinking we rely on when we don't really want to think. It's a sign of procrastination, not productivity.In recent years, however, it's become clear that daydreaming is actually an important element of the creative process, allowing the brain to remix ideas, explore counterfactuals and turn the spotlight of attention inwards. (That's why increased daydreaming correlates with measures of creativity.)
To imagine is only the first step. School as we know it may be pretty useless for encouraging raw creativity, but it is all important for, first, giving us the necessary background to imagine in a concrete and useful way, and, second, teaching us the habit of following through on our ideas and turning them into products we can share with others.