Doodling in your textbooks is bad! I do not recommend it. At all. However, let's say you have to doodle in your text -- like, if you don't, you get in trouble. Then, by all means, please deface the book in the most entertaining way possible.
Above, you can see Chinese revolutionary Sun Yat-sen getting the Mario treatment in a Japanese textbook. And below, you can see schoolbooks from throughout Asia that have been defaced in a variety of creative ways. A few of them you might have seen before. But all of them, amazingly, are penis-free. That's right, you don't need to doodle penises to be entertaining when you deface your books. It doesn't hurt though.