Kurokami Seiso-chan Monogatari -Kareshi ja Nai Hito de Iki Kurutte Ecchi Daisuki ni Natte Shimau Joshikousei-

The protagonist, Kurokami seiso-chan (Ayame) is a high school student who aims to attend a national university together with her boyfriend. One day, Ayame happens upon her classmate having sex with his sex friend, and thought "Is sex really different with other people?". He then said "If you're curious, then won't you try doing it with me?". She refused the invitation at first, but as she remembered the two having sex so differently than the way she has sex with her boyfriend, she can't help but get curious. That night, as she was studying with her boyfriend, she was rearranging her schedule in her mind. "If I add 2 hours to my studying time per day, then I can have some free time on weekends......." As she doesn't want her studies for her entrance exams to be affected, she gradually increased her study time per day, and Ayame, now with some time for sex, finally did it with that classmate, where she experienced pleasure enough to make her gasp for air. "How was it?" "It was amazing. Really amazing. It was totally different." Ayame, learning that sex with skilled men is totally different, went and sought after various sexual pleasures with her senior who is known as a sex technician, as well as the teacher in her cram school she's interested with. [Translated from Official Site]