DearS Vol 1

From the Tokyopop edition

Aliens have landed on Earth and are now a normal part of society. These beautiful beings have been given the name "DearS" and are trusted and welcomed by most humans. In order for the "DearS" to learn Earth's customs, they are sent to random high schools to "home-stay." When Takeya helps a DearS in his school, she calls him "Master." Thus begins the humorous life of Takeya and his sexy alien follower, Ren, who tries to figure out the wacky customs of this place called Earth!

Source: Anime News Network Encyclopedia