A Female Teacher's Dangerous Lust(1996)

Yuko becomes a teacher at an all-boys school. She just spends her days going back and forth between her home and school in vain. And then there was her 30th birthday, which she celebrated alone. At a boutique she stops at to relieve her loneliness, she meets Seiichi, a young clerk. Coincidentally, she is revealed to be the lover of Hideko's younger sister, and before she knows it, they begin to spend sweet nights together.