S1E39 : Sleepy Serene Springtime Classroom

During math class, Kuribayashi and all the students fall asleep, save for Ataru and Mendo. Lum arrives and finds out that a tiny old man and girl are responsible for this magic. The tiny girl then uses some more magic to have a party with Lum, Ataru, and Mendo; until the principal arrives with some parents.