Hitozuma Kyoushi wa Moto Mahou Shoujo ~Oshiego no Boku ga Otona no Onna o Netori Otosu~

Who knew? Wife, mother and schoolteacher Nanako Hikawa used to be a crime-fighting magical girl!
Back in her youth she cast spells and saved worlds as Sapphire of bishoujo team "Justice Pirates".
15 years after they destroyed the stronghold of an evil organization, the Mad Professor's grandchild Tsutomu discovers that his teacher was a member of this group. Using the devices that he found in his grandfather's warehouse, he plans to make Nanako his sex slave.

Nanako (a.k.a. Sapphire) unknowingly ingests a hardcore aphrodisiac, gets bawdy with a Tsutomu in her marital bed, f*cks her student in class, gets pregnant...
Husband be damned! Find Nanako on campus and see how much she enjoys breaking the sacred vows of marriage and teaching.

[Edited from DLsite English]