Go to Google and type "potato gun". There will be about 500,000 results.

You are limited to compressed air (the bungies will do very little.) Most potato guns use some sort of combustion. An example:

http://www.mshamash.com/spud/spudgun1.html

For compresed air see this:

http://everything2.com/index.pl?node_id=1091027

I am absolutely shocked that your teacher can get away with this assignment, since potato guns have a tendency to explode. Administrators have a tendency to explode around such projects, as well. They are fun, but they have almost zero pedagogical value. This is the same as the damned "egg drop" that high school physics teachers love to assign to fill time. (Sorry for the rant. As someone who then has to teach these same students everything they didn't learn one or two years later, I get a little tired of having to hear about how all they ever did in high school physics was build potato guns and set up laser shows.)