Omi Sensei no Binetsu

From Stiletto Heels:

Omi Tsukasa, who still holds a flame for his close friend and first love, Su-chan, becomes a new teacher assigned to his old school. There he spots Su-chan's doppelganger, a student named Sugimoto. Upon finding out that Sugimoto is Su-chan's nephew, Omi starts having an interest in him, much to the annoyance of his target of interest.