As Open Day season is approaching, I thought I'd kick off with one of my own 
Went to visit a local independent for 4+ entry. The tour guides (year 6 pupils) did a lovely greeting until we got to the reception and year one classes, when they announced loudly 'and this is where the boys and girls just play most of the day, because they aren't old enough to learn much yet!' The teacher's face was a picture...
At the same school we accidentally walked into a barney between two of the staff over a lunch rota, and the headmistress had to intervene.
I'm sure there are much more extreme experiences out there!