Besides other good things about shifting teacher's work from lecturing to actually spending time with kids that were mentioned here, I would like to point out that some teachers are really bad lecturers. My most vivid impressions are from early collage math courses but it is as valid for high school as well. The professor can lead the lecture at million words a minute constantly erasing the board so you don't have the time to copy the material, let alone let it sink in. The professor can have heavy accent, so you spend most of you attention just trying to understand his words.