Otona no Shoumei

From Shoujo Crusade: Tsubaki always appears experienced, but actually, she has never had a boyfriend even though she's already 16, and she's a real baby. She wants to quickly increase her love experiences, and become an adult! One day, she got called out by her math teacher, Shiina-sensei, and she was told that she'll have to take supplementary lessons. All alone in the math room with her sensei after school, Tsubaki's heart beat like crazy when she saw Shiina-sensei's facial expressions that were different from usual...?