This Ninja Is Obviously Not Strong but Really Hard to Kill - Chapter 380: Nightmare

Collapsed without warning, and upon awakening, he found himself back in his childhood.He thought it was a blessing from above, but unexpectedly, tumultuous changes ensued, shrouded in mystery...

I lived for you in the last lifetime, everything I did was for you, yet you wanted me to die; so, this time, life is mine to live! This world lied to me, betrayed me, and humiliated me, and I am back to change everything!

- 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.

Everything in the world can be repaired!

- Description from novelbuddy