I don't get it. If he wanted to live a normal life away from his past years of fighting, then why did he enroll in a combat school? Why didn't he enroll in an academic school, magic school, alchemy school, medical school instead?
(The author forces conflict to generate plot.)
I seem to remember several other manga with a similar story. The ice magician, the child soldier teacher/bodyguard, the student who's lived thousands of years goes to mage school, the student at the demon lord candidate school, the adopted human who attends the demon school.