The New Gate (LN) - Vol 10 Chapter 3.3: Chapter 3 Part 3

The eldest daughter of the Li family is beautiful and sassy, ​​she took space materials to flee famine, and after going through many difficulties in war, disease and epidemic, she united with her family to regain the glory and harvested a beautiful boy.

- Description from novelbuddy

Wang Xiao was compelled to descend from the mountain by his master, with his intrinsic energy cloistered, and a lovely woman erroneously believing him to be her betrothed. In a bid to safeguard his mistakenly assigned fiancée, he resolved to remain in the school teeming with enchanting beauties. With the same intent to defend his intended, he managed to transform the currents of power within the city.

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

- Description from Novelupdates

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.