April fool's jokes really aren't my thing, but I've been doing one in my classroom since I started teaching 19 years ago. (My cooperating teacher shared it with me when I was a student teacher.)
Below is an old report card that my district hasn't used in ages and ages. It looks nothing like the one we currently have, which is a good thing, considering that it's so poorly written and is part of our April fools joke. I've added some grades and made some comments. They're not altogether complimentary as you can see.
When there's school on April 1st, each kid gets to take one home. First, I make sure to adequately demonstrate how to pull this off. Presentation is key. The pouty face, the slumped shoulders, the hanging head...you get the idea. I also make sure they let their parents off the hook in a timely fashion. "April fools!"
If you'd like a copy, click on the picture. I do think it's best that it looks nothing like the one parents are used to. It's more easily identifiable as a joke that way. I don't see why this couldn't be used for other grade levels too.