I don't know this is a "gaffe," exactly. But at my city high school graduation, 5 students were not permitted on the stage to collect the diploma because they were wearing sneakers. Not appropriate attire for such an event.

The middle school behind me did a simulation for a lock down. Teachers knew before hand, students did not. Had to call it off as one classroom went totally hysterical and couldn't comprehend when the teacher tried to explain it was a drill. The poor kids were terrified, all squished in a closet, with the desks all propped at the classroom doors.