Manga/Manhua in Which the Main Character is a Teacher.

1. World Teacher: Isekaishiki Kyouiku Agent.

A man who was once called the world strongest agent ended up becoming a teacher after his retirement to train the new generation of agents. After many years of training his disciples, he is killed at the age of 60 by the ploy of a secret organization and was reincarnated in another world with all his past memories. Though he was surprised by the magic and the strange species of that world, he adapted very fast to his condition as a newborn and took advantage of it. He acquired special magic and gained a massive amount of strength thanks to his tight discipline, in order to reach his goal: Resume his career as a teacher which he left halfway through in his previous life.

2. Weak Teacher.

The special discipleship of Moorim's best assassin, "Gwi-Myeon-Dok," will begin.

3. The Reincarnated 「Sword Saint」 Wants To Take It Easy.

Alta is the youngest knight in history and the reincarnation of the legendary "Sword Saint". He is dispatched to the school to protect Iris, the "Sword Saint Princess" who is the next king's candidate, but "Please fight with me seriously", but Iris, who is supposed to be his escort, suddenly challenges him to a fight...!

4. Arafoo Kenja No Isekai Seikatsu Nikki.

Satoshi Osako was a player of the VRRPG 『Sword and Sorcery』 who defeated the Evil God and died for his troubles. When he awoke again he found himself in a strange unknown forest. Furthermore he possessed his avatar’s body and abilities. In other words, he was a cheat. Can a 40 year old DT, live peacefully in this world?

5. Tale Of A Scribe Who Retires To The Countryside.

A scholar who failed the national exams is picked up by a martial arts master and given the task to record his life into a book. Through their journey, Son Bin meets many great people and soon finds himself following the same path as the master, as the Martial Arts King.

6. Cultivator Against Hero Society.

A powerful cultivator unexpectedly time-traveled into the modern world with heroes. In this small world, the fight between cultivation and superpowers shall begin...

If you know more such manga/manhwa/manhua just mention them in the comment section.