Ushiro no Seki no Aitsu

Koda is often teased and tickled by two friendly female classmates every day at school. One day, he accidentally arrives late for the cleaning duty that the three of them were in charge of. As a punishment, he ends up being tickled relentlessly. Despite being tickled until he was worn out, for some reason, he found himself getting excited every time he remembered the tickling... The next day, the two girls discover, of all things, that he had been aroused while being tickled. With his weakness exposed and his curious desire to be tickled mingling in his head, he finds himself in the girls' room, stripped down to his underwear. Koda, pinned down to the floor by the two girls, is... (Source: DLsite, translated)