Christy certainly a trip to Walt Disney World is worth missing a few days of school.
As a teacher, I recommend you look into
Disney's Youth Education Series. Registering your children for a program or two in this program might go a long way in convincing school officials to allow you to take your kids out of school for a short time. Topics available include American history, zoology, marine science, leadership, physics, and more.
The best places to to look for samples of school excuse letters is probably non official Disney sites such as the mouse for less or mouse buzz. Both these online spots offer helpful example school excuse notes.
I hope these resources will help you be able to pull your children out of school for visit to Walt Disney World!