I always took it personally when my teachers called my textbook art "doodles," but not as personally as these creative students in Japan would likely take such an underestimation of their work.
You shouldn't ever deface your textbooks, but if you're going to do it, do it right and make the doodles true textbook art. This gallery of various Japanese students' art is as impressive as it is geeky. Especially impressive is the portrait of Chinese revolutionary Sun Yat-Sen turned into everyone's favorite portly plumber, Mario. Lots of popular culture is reflected in this textbook art, including Pokemon, baseball players and even turning a mathematical graph into a posing bombshell.
Compared to my meaningless doodles in elementary school, these images are the true examples of textbook art.