Have You Seen The Rain? Sasamekikoto : Season 1 Episode 12

On a rainy day, the gang discover a hidden note in a book referring to a certain thing, so they decide to search for it. After some investigation they find a key to a locker, but are reluctant to go to the boy's locker room, so Sumika gets Masaki to go for them. The locker contains another note mentioning a German person. With Azusa's help, they make the connection to the pictures of German musicians in the music room. This leads them to the creek (which in German is 'Bach'), eventually finding it to be a prank, but having fun all the while.