Pulling off a prank this elaborate says something about the person who does the prank. For these parents to go through the whole process of making a fake letter, and then mailing it, borders on psychotic behavior. It's so elaborate that no one would ever suspect it as a prank. That's what's so good about it and horrible about it. I can only imagine how insecure this kid was about finishing their schooling. The parents played on that insecurity like it was a fiddle. A fiddle made of broken dreams.
If you want a prank that will surely fool someone, always go with a letter prank. It's easy to find letterheads that companies or schools use. Do a quick brainstorm and figure out with one of your friends or family members you can do this to.