The Melancholy of Haruhi-chan Suzumiya S01 E14 That's not the face of a main character!

Haruhi and Kyon have a small battle involving balloons and blow-guns. However, when Mikuru wanders into the room, she is used as the unwitting victim of a balloon-decorating contest, with Itsuki as the judge. Meanwhile, Yuki finds some extra un-inflated balloons, and decides to take them home.