Welcome to Gately Comprehensive, a secondary school where no two days are the same. It could be anything, from the swimming pool being filled with jelly, a stampede of zebra through the gymnasium, right up to an alien invasion.
As the put upon teaching staff it's your job to take care of these problems, while ensuring you still cover all your classes, keep up with the marking and avoiding being volunteered for any extra-curricular activities. And all sorted in time for a pint down the pub after school, of course.