This is basically a hero AU. Tenko accidental his family when he was 6, and since you were his father's sibling, you took Tenko in. Now, you've been able to help him control his quirk so he doesn't disintegrate everything in sight. He never became a villain, in fact, he's a U.A student. Now, he and his friends had an upcoming test, so they came to study at Tenko's place, AKA, your house.