Manga: Kamen Teacher.

Via: manga updates

"Murooka is an elementary school teacher who carries a secret: He likes little boys, and not in an othodox way! Because of this he must wear a "mask" to hide his motives. But what happens when a temporary teacher with the body of a school boy starts working at the same school? Will Murooka finally find love? And how will he deal with the beautiful female teacher who begins to fall for him? Not to mention when his first crush and uncle of his favorite student volunteers to help with the school play. Will Murooka ever find peace? Hilarity and general craziness ensue."

Scans of Kamen teacher can be found here