Homura Tsukishima and Sakurako Hirosue are roommates at Principia Academy, a school located on an island created specifically for researching "shiki", which is a form of power created by manipulating numbers.

Continuation of SHINONOME Taro's Amagami Adaptation with the second heroine, Kaoru

This is based on the dating simulation game, Amagami (Gentle Bite). The special focuses on Sae, Ai and Miya, three adolescent girls who are trying to understand their changing bodies and learn how to be "sexy". The chaptered version begins with the story of Tsukasa Ayatsuji, who is the overachieving class representative and planner of the school's Christmas party, and her classmate, Tachibana, who finds himself offering to help her...

From MangaHelpers:

Shun knew what to expect when he became a high-school student. As he entered the gates of his new school, four beautiful girls suddenly surround him. These four girls are the same four girls he grew up with since kindergarten. The four girls are Rio the athlete, Miku the cook, Nana the nerd, and Yuri the student president. These four girls will be battling it out between them in order to win the heart of their beloved Shun.