Dekisokonai no Monster Trainer

In a world where everyone has a monster crest since birth, only a select few rise up to become Monster Trainers. Reyn Erhardt, a slime trainer, the lowest of the low, cares nothing for status and just continues his daily training regimen together with his personal slime, Pem-Pem. In a school dominated by monsters as powerful as dragons, those who cannot hope to rise to the top take their frustrations out on the weak, and Reyn and Pem-Pem make for easy targets. Or do they? Source: Baka-Updates