I Got a Cheat Skill in Another World and Became Unrivaled in The Real World, Too

Yuuya Tenjou has been bullied at school, neglected at home, and consigned by his own parents to the unwanted-child shelf since he was small. The grandfather who left him a half-collapsed house on the edge of town also left a sealed door at the back of it that turns out to lead, when opened, into a magical continent. The continent is generous to first-time visitors. The skills Yuuya brings back home — combat instincts, healing magic, a body that has been steadily levelled up in the other world — are still active in the real-world high school he keeps having to return to.