If anyone's curious, the title of the manga carmozo is thinking of is probably:
Hazure Skill "Kage ga Usui" o Motsu Guild Shokuin ga, Jitsuha Densetsu no Ansatsusha.
The titles of the mangas that Kefrayba suggested are probably:
1. Saikyou no Mahoushi no Inton Keikaku (this is a novel series that has 2 manga adaptations)
2. Ouritsu Mahou Gakuen no Saikasei
Other manga with a similar premise:
1. Maken Tsukai no Moto Shounen Hei wa, Moto Teki Kanbu no Onee-san to Issho ni Ikitai
This manga follows a child soldier who joined the Hero's party, but was kicked out because "his methods were too brutal". After getting kicked out, he meets a former Demon General (an onee-san type character) who was also kicked out from the Demon King's Army for allowing the Hero's party to progress too much. After their initial reservations, they both realize that they no longer want anyhing to do with the war, so they decide to find a place to live together.
2. Oukoku no Saishuu Heiki, Rettousei to shite Kishi Gakuen e
While (admittedly) I haven't really read much of this, it shares a similar premise: guy hailed as the strongest secret weapon defeats a powerful demon, finds his life lacking, decides to attend school, is the strongest in the school, is treated as a bad student bcause of ranks, eventually rises to crush the arrogance of the nobles (I think).