Kawahara Kazune is one of the greats--one of the people who built the shoujo romance genre with classic titles like High School Debut, but also the mind behind the more modern off-beat awesomeness of Ore Monogatari. Sensei! is an excellent example of the more realistic end of the scale in student/teacher shoujo romance. It has a couple of annoying arcs, but overall I found it memorable, well worth a read if you have time for all those chapters.