Need a good pocket sized mini cannon

[Insomniac]

Does not matter. They are fun novelties to build and use, but they are still dangerous if you are not very careful. The penguns may not pack a huge punch, but they are still guns and should be treated as such. Would you take a handgun to school?

[Fnord]

Even if the shop teacher is cool with it, beware of the ones who are not.

I'd recommend something rubber-band based for school, since you typically won't get in a great deal of trouble for it. About ten years ago I had those "pen tube with a rubber band" things down to a science. I made darts out of stirring straws and round toothpicks. With a heavy rubber band you could stick one into thick stage curtains from the across the width of a basketball court.
However, you could easily put an eye out with one, so never shoot it at people.

Then I made a semi-auto bb shooter with the same principles. It only stung when it hit you, but was quite a bit of fun for that reason.