Trying to find the name of this magic school manga. The male lead ends up a teacher after pressed into service by his older sister (not by birth)/mentor/surrogate mother type character. Appears weak, but is really strong etc. Was some sort of child soldier.

Thanks!

I kinda think you are referring to Mahou Sensei Negima!. It sounds sort of right, but I don't think Negi was a child soldier...

As a former scanlator of Negima, it is for sure NOT that series

A just ruler amongst tyrants

Sounds like Roku de Nashi Majutsu Koushi to Kinki Kyouten.

Glutton13 is right, thanks!