Houkago is more than just a shoujo-ish adaptation of a mystery novel. At first glance, the characters seem bland, predictable, and especially the teacher is the prototypical flawless main interest. However, with each new revelation, his characterisation becomes darker. It is his human side that made me root for him, despite his faults. My advice: Don't go into this thinking that there will be some miraculous love story, don't expect some clever trick. Just enjoy how your evaluation of the situation, your perception of the characters changes. Achieving that sort of subtlety is more difficult than coming up with a good locked room murder. Kudosto the mangaka.