Hmmm... Although you can relatively easily do the ninja out fit quickly. (Balaclava, Black Pants, Hoody) I'd recommend going as something slightly different. Spice it up by not wearing black, (white ninjas are cool~ just look ast storm shadow). Make a Splinter cell nightvision goggles out of spray painted toilet paper tubes. Walk to school wearing normal clothes, but be able to strip out of them in 2 seconds into a ninja suit. (Ninja suit under clothes) Wear only a pair of black shoes/sandals + black shorts, and paint your entire body black. Spray paint some sweats the color of your schools lockers. Don't even show up to class, and ask why you were counted absent.
Cheers!