This is probably no help to Jkid, but I started reminiscing and now I wish to share.
I used to draw on my pants all the time, especially in 8th grade when I was in a public shool. At one point in time I had a peace sign on every pair of jeans I own, smiley faces on most of them too, as well as detailed pen-drawings usually of landscapes, boats, or skeletons in deserts. One time I put an anarchy sign near the opening of my front pocket and then thought "I'm not really an anarchist, I shouldn't have put it there," but then I realised how not bad it looked and thought "I could live with this," And other kids were always suprised, jealous, and/or in disbelief of the fact that my mom didn't get mad at me for drawing on my clothes.
Helpful hints- Sharpies and fairly perminent, ballpoint pens will wash out eventually, magic markers last a suprisingly long time, highlighters are weird, but none of these are imposible to get out. Drawings can be altered to look like other things if you accidently do something wrong. Draw with your pants already on if possible so that the image looks right.