@Avendora According to a SimGuru, George I think, prom is every week so that your sims will have a chance to go regardless of what lifespan you have aging set to. You don't have to go to every prom. I agree though that giving us the option to control things with the calendar would make the calendar much more useful.
As for exams, that too is most likely the way it was designed for the same reason as prom, but again, giving up more control (I play with aging off) would make the game so much better).