Furyou Shounen-kun, Ecchi na Onee-san ni Ijiwaru Saremakuri

Kou Saejima is a juvenile delinquent boy, and Mei Aizawa is his senpai who's popular in school, across all grades.
Saejima, intending to skip class headed to the infirmary, but there he caught Aizawa giving a male student a blowjob. She tries to make him take responsibility for interrupting their now failed session and he resists wildly at first, until his arms suddenly get restrained...
(Source: DLsite garumani, translated)