Yongsa Dongsaengeun Academy Goinmul

Ruud Denatos' life of living in the shadow of his talented older brother Aesir, a genius dragon slayer, ends when Aesir is killed in a brutal battle. After the fight, a hidden dragon lord appears and sends Ruud to a past where Aesir doesn't exist! In order to find answers and his older brother, Ruud takes Aesir's place and wows the school with his skills and knowledge. But will Ruud continue to impress the academy, or will the lackluster skills of his past catch up to him?
(Source: Tapas)