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.<p>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.