In my first year of high school, my class decided to play a simple prank on our English teacher by all laughing when he faced the board, and then stopping every time he turned around to face us. After a few minutes of this, he just left the classroom without a word.
We all sat there, confused until a few minutes later the assistant principal comes in and explains that we’ve really upset our teacher; he made us believe we’d seriously messed up. Then our teacher walked in and pretty much went “Gotcha!” He had our respect from then on.