Gakuen Polizi (学園ポリーチェ) is a yuri manga by Milk Morinaga. It began its serialization in the Comic High! manga magazine in May 2012, and ended in June 2014. It is complete in two volumes. Seven Seas has licensed it for English release.
Ever since she was a child, Sasami Aoba dreamed of becoming a defender of justice like the magical girls and sentai rangers she sees on TV. Years later, Sasami has fulfilled her dream and become a police officer. Her first assignment is to infiltrate Hanagaki All-Girls High School and ferret out any troubles she finds. However, on her first day of posing as a student, Sasami is shocked to discover that an apparent book thief, Sakuraba Midori, is actually another undercover police officer. What's more, Midori claims that she is the one in charge of infiltrating the school!
Official English site