Manga where the protagonist gets letters of recommendation from powerful figures, and changes his age, to go to a school

Question

I am trying to find the manga where the main character gets a letter of introduction from the Spirit King, Demon King, and the Dragon King.

• He also uses his power to change his age so that he can get in the school.
• He had a supposedly legendary bird to get the letters.
• He went to school with the Demon King's daughter who at first he thought was a son.
• The introduction letters caused a huge commotion between the teachers and principal. -On the first day of school he summoned the spirit king, which broke a barrier.
• He was a guy who fought the demon king after the hero lost.
• Magic school.
• Everyone had human like features.
• He joined the school with a girl from an aristocrat family.
• He knew ancient magic but wanted to learn new magic.
• He was in a tower studying books for a few centuries.
• Seen by elves as a heroic figure of some sorts.

I don't remember if it was in color or black and white.

Answer 1

This is Monogatari no Naka no Hito (A person in the story)

A young noblewoman once dreamed of becoming a magic user like in the old tales, but her time in magic school has dimmed her passion. One day, a mysterious man comes to her rescue in the forest, using magic like in the old stories. Despite the fact that he is clearly skilled in magic, he is thrilled by the idea of learning new techniques. Since the only way to do that in this day and age is in magic school, he alters his appearance to school-age and decides to accompany her back to school!