Very delicate and nicely original take on the old trope "high school student + teacher love", this "Sensei Sayonara" positively surprised me for its beautiful script, the good to great acting performance by the main cast, and a totally decent production value / "packaging" (direction, photography, music commentary etc.).
Hard to explain the originality of this little drama (and most of all, why you might wanna give it a chance even though you might be turned off by the premise) without spoilers, so here we go, consider yourselves warned and if you don't wanna get any of those, stop reading now!
The afore-mentioned originality lies in the fact that the drama constantly juggles two different storylines: on the one hand, in the past, we get to see the beautiful-yet-tragic love between the ML (then a high schooler himself) and his homeroom teacher (played by Kita Kana who's nothing short of brilliant here, her last scene literally gave me goosebumps!); on the other hand, in the present, we see the sweet (and rest assured, completely chaste) love between the FL and the ML, who's now become a homeroom teacher (and a widow). In both cases it's the student who initiates everything, and it both cases the relationships are portrayed in an absolutely pure, not-in-the-least naughty way.
So why don't I give this a perfect 10? Well, honestly, Kita Kana and Watanabe Shota both did really well, whereas the rest of the cast (even more so in comparison to the Himalayan heights reached by those two) was good but nothing more (most, like Hayashi Meari, are pretty young so there's still a lot of room for improvement of course, ganbatte kudasai!)