You know it.
All you need is a big, ol' round necked t-shirt and a pair of scissors. Basically you cut away everything that is not part of the cape. Be sure to keep the neck attached! Set the sleeves aside in case you want to accessorize.
You will have a circle with a cape hanging down your back. If you are finding this hard to picture, here's a step-by-step tutorial that also includes an option for making a Velcro closure and a wristband!
Just imagine what kind of awesomeness could happen when 450ish 8th graders are wearing these cloaks of goodness.
You could probably find a better way to fancy these capes up, but I like the bargain basement plan. That way more kids can participate, and I've always loved Captain Caveman's seemingly inadequate scrap of material.