Urusei Yatsura: The Shougaibutsu Suieitaikai

The story is from manga volume 23.

On a very hot summer day in Tomobiki town, Lum, Ataru, and the rest of the gang are looking forward to using the school's pool. But the teachers have other plans: setting a ludicrous obstacle course between the students in their swimsuits and the pool... naturally this ends in complete chaos.