Today’s strip can be seen here today. (If you’re reading this later, it’s the September 17th strip I’m curious about.) Not being a student of art history, I don’t know what a student paying tribute to Francisco Goya would do to shock the grown-ups so badly. I did find an image of Saturn Devouring His Son on Wikipedia, and that seems to be the most likely answer. Can someone who knows more than I tell me if I’m in the ballpark?
That’s what I thought when I saw today’s strip. Among the daisies and puppies and all, it could be quite disturbing. Probably drawn by Caulfield. 
I thought of the Naked Maja.
What confused me was that all of the visible pictures were perfectly normal kid-drawings. I had to stare at the comic for a couple of minutes before I realized that the offending drawing was around the corner, not visible.
I think this is one of his more known works. (note: jpeg is 863 x 618 pixels, 483k)
I just went looking for possibilities, too, on seeing that strip. Personally, my guess would be the Witch’s Sabbath. The Naked Maja probably wouldn’t be it… While a naked lady would be somewhat surprising at an elementary school, there are enough nudes in art that it wouldn’t particularly be a tribute to Goya in particular.
Now tell me… How many other comic strips would not only work in references like this, but actually inspire people to go looking for the answers? This is one of the reasons I love Frazz.
After a Wikipedia search for Goya, I cast my vote for Saturn as well.
I think it was a nod to the Calvin and Hobbes’ cartoon of Saturn Devouring His Son.
I don’t remember that one. Was it a regular C&H strip or was it in one of the books?
Goya also did a series of horrific illustrations entitled “The Disasters of War.” My vote’s for “Naked Maja” though. It’s pretty erotic, even taking into account the vast amount of nudes in classic painting.
It was surprising enough at the time that Goya got hauled before the Spanish Inquisition.