Outdoor School - S1.E7

Rin and her classmates all went on a class outing to the mountains. While staying there, they play mischievous pranks on Aoki. The episode ends off when as the class were about to leave, Kyoko went to wake them up, finding the three of them, Rin, Kuro and Mimi, fell asleep with Aoki under a tree. Apparently. Aoki's pants were soiled, actually due to Rin's saliva, but the three of them made it look as though he ejaculated on his pants.