Dekisokonai no Monster Trainer  

できそこないの魔獣錬磨師〈モンスタートレーナー〉

In a world where every individual possesses a unique monster crest from birth, only select individuals become Monster Trainers. Among these trainers is Reyn Erhardt, who specializes in slime training despite his low status. Despite the powerful dragons that dominate the school, those unable to compete take their frustrations out on weaker students like Reyn and Pem-Pem.