I think I'm going to draw a butt on my friend's test
Why would you do that?  It won't do anything to help you
I wanted to teach that guy a lesson.
Drawing on someone else's work is very embarrassing to them, and it can cause serious consequences.  Whatever your friend has done to upset you, you shouldn't be trying to sabotage his academic standing as a result.
I think I should do it. We'll see when I finish the test.
I think it's better to draw this on something less important, like on a friendly note to him. Tests are really important, do you know how he might feel being distracted and embarrassed from this?