Kodomo no Jikan

At the request of their mothers, a high school girl agrees to tutor three younger boys from her neighborhood.

With the start of the very first tutoring session, she quickly learns that what the boys want to study isn't part of the standard curriculum: soon she's naked, bound and gagged, while the boys gleefuly play with her helpless body. At first she tries to break free, but her struggles quickly turn to moans of pleasure.

(Source: MU)