Yokota is a lonely high school teacher who has lost his only family member. Then a new teacher, Miyazawa, a former student of Yokota's, arrives at the school. He is the opposite of his serious and inflexible self, and is a popular person who is well-liked by everyone. He is also very fond of him, saying, “I became a teacher because I admired you so much." Then, his usual good-natured face changed drastically! He says, “You're mine now, aren't you?"

I died in an unfortunate coincidence involving the Goddess, and as an apology, I was given the skill to use magic and reincarnated into another world. However, while the Goddess was away, magic had declined in that world, and now there are no humans left who can use it...? So, being able to use magic makes me a cheat, right!? While I'm at it, I might as well make full use of my cheaty magic and enjoy my second life to the fullest! (Source: Kadokawa, translated)

https://www.cmoa.jp/title/297453/