Urban Seduction Adventure - Chapter 37: She’s Still a Virgin

When The Cute Kunoichis Spied on Their Teacher's Secret is a light novel written by Kazuma Kamachi and illustrated by Ryuushi Tajima.

The story takes place in a world where Japan never ended their isolationist phase, and agents from foreign powers are always threatening to steal the country's advanced technology. In order to combat this threat there has been a resurgence of ninjas that use Strategic Martial Arts, techniques that combine cutting-edge modern technology with a traditional foundation of martial arts and ninjutsu. The story follows Souha Sugiyado, a 17-year old ninja instructor and former ninja prodigy who was forced to retire from active duty by the wounds he suffered in his last mission. Souha's four students are all kunoichi who have all gained the rank of Elite Ninja, the highest official rank available to ninja. However, one day two Shogunate ninja approach him with the news that his four students have gone rogue and have stolen a portable modular nuclear reactor, and thus they ask for his help in bringing his students to justice.

One day, Katsuragi Daichi and his classmates found themselves in a different world. The god who summoned them to another world divided its power and gave it to them, saying that it wanted Daichi and the rest to defeat the Demon King as heroes. However, Daichi alone was given no abilities. And sure enough, Daichi was thrown away and used as a decoy in a demon’s den. Eaten by a demon, Daichi had lost consciousness and thought he had passed away, though when he came to he noticed his statuses had skyrocketed. He had obtained『Revenge of the Resented』. Every time he died, his abnormal ability powered up.

Childhood sweethearts.
The most popular person in school is my childhood sweetheart.

- Description from Novelupdates