Urusei Yatsura S01 E39 Sleepy Serene Springtime Classroom

"Grab her horns and she might just become your darling!"

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.