Eiyuu Kyoushitsu

In this anime, the main character is extremely strong, while he also tries to hide his real strength so that people do not know, the main character is also friendly, trains a lot and tries to help whenever possible (the main character, as for me, these qualities stand out perfectly, while he is not very reasonable because he solves everything quickly without thinking about the consequences and bonuses, the main character's boss is the king, just like here, and the main character is also studying at the academy). In my opinion, it is perfect, I have described these qualities and factors that stand out strongly and are very similar to this anime. read more 

In both anime main character is overpowered hero and stop their hero career but still do something similar as what they did. In yuusha yamemasu, mc quit heroing and join demon army, in eiyuu Kyoushitsu mc joins hero school. They both have fights but also slice of life moments. 

Both series blend academy life with an overpowered past hero concealing their identity while focusing on the importance of forging strong friendships.