Lily Lily La La Land

There exists an all-girls school exclusively for up-and-coming young ladies where a system of exchanging keys to one another is acknowledged as a form of romantic confession. One fateful night when Emi Kewaki walked past the rumoured mystery-filled Room No. 101, a black shadowy figure came out from the room and the very next day recieved the key to the so-called forbidden room. A struggle for the key (and it's bearer) by the school's five top young ladies then ensues!

(Source: Kadokawa, translated)