Hmm...thats funny. Some 7th graders at my school just had that assignment..um...try making it out of wood. Ok I think that was no help..sorry =]

The site in the source has several designs that you can make for free, as well as links where you can buy kits (or at least look at them). Probably one of the free designs would work fine. There's a really simple one that has a picture several posts down, and with a link to the plans.

If you need to build one that is more authentic, read through all the posts and learn all you can about how catapults were made and hopefully you'll find something that will spark your imagination.

Feel free to contact me if you need more ideas.