Tonari no Hogosha-chan

After twelve years at girls-only schools, Riko is ready to start a new life at a co-ed university. One day, she volunteered to become the guardian of Mizuki, a junior-high boy living next door whose family was moving away. The first time dealing with college guys felt scary and intimidating. It's so much more comfortable talking with Mizuki, who is just a gentle boy. Wait, she's supposed to be the responsible adult here, no impure thoughts allowed!

(Source: Kodansha, translated)