In a very nice and respectful way, of course!
Buy one of those HUGE eggs that are probably on sale right now. Tell your students that they will be filling the egg with egg-stra special letters that they will write to their principal. (We also egged our assistant principal!) Of course, we used this as a mini-writing lesson and reviewed some egg-cellent word choice that they might use in their letter, such as the ones listed below:
I placed all the notes in the egg, sneaked into the office of our principal and assistant principal and officially “egged” them.
Go egg your principal before Easter! :)