My students forged the notes. I turned them into a lesson plan.
Excuse notes are a great example of why you need verification. Anyone can write one for themselves, and give it to you without proof it really came from a parent. That's a classic man in the middle attack.What you really need is some kind of certificate for each parent, allowing you to verify the note really came from the parent. That's why self-signed certificates really aren't worth much -- anyone could have created them.