Practical Exam
The Chemistry practical exam is held as part of the AS and A level exams. Each student gets a tray full of chemicals and glassware, and most excitingly, a brand new apron, that both protects the user and ties up properly at the back.
As there aren't enough labs for all students to take the exam at once, some must wait until the afternoon. So that no unfair advantage is gained, half of the classes are locked in the Sports Hall's classrooms for the day, with their mobile phones and internet access confiscated, and are escorted to the dining room for lunch like prisoners. Which they effectively are.
The 2006 A-level exam was interrupted by an unscheduled fire alarm, but rather than being allowed out to the Astroturf the students were penned in by the labs. Where all the explode-y chemicals are. Good job it was a false alarm!