Dekisokonai no Monster Trainer

Dekisokonai no Monster Trainer summary:

In a world where everyone has a monster crest from birth, a select few rise to become Monster Trainers. Reyn Erhardt, a slime trainer, the lowest of the low, cares nothing for status and just continues his daily regimen of training for his personal slime, Pem-Pem. Yet in school dominated monsters as grand as dragons, those who cannot hope to rise to the top take out their frustrations on the lowest ones, and Reyn and Pem-Pem make for easy marks. Or do they?