Seirei Gensouki - Konna Sekai de Deaeta Kimi ni (MINAZUKI Futago)

From J-Novel Club: Meet Rio: a callous orphaned boy living in the slums. At only 7 years old, he realizes he’s actually the reincarnation of Haruto Amakawa, a Japanese university student with a tragic past. While still reeling from this shocking epiphany, Rio also comes to learn that he possesses extremely potent magical abilities and uses his new powers to solve the kidnapping case of a little girl. His good deed is acknowledged, and he’s rewarded by being enrolled in a prestigious academy for noble children.