A K'nex catapult.

Pros:
-easy to build
-sturdy
-portable
-good range, power, and accuracy
-easy to load and fire
-looks kinda like a real catapult

Cons:
If you find any I will post them here. Just PM me or leave a comment.

Step 1: Parts List

So short, I almost didn't make one. This does not include ammo (1+ dark grey connectors).
If you do not use all of the parts when building, you probably did something wrong.

Connectors
5x dark grey
1x green
12x purple
1x white
4x orange

Rods
2x green
7x white
8x blue
3x yellow

Other
2x blue spacers
1+ elastics

Step 2: The Frame

1. Make these.
2. Make these.
4. Put them together.
5. Add a yellow rod as shown.

Step 3: The Throwing Arm

1. Make these.
2. Put them together like this.
3. Make these.
4. Put all parts together.

Step 4: Final Assembly

1. Take the throwing arm and add a blue rod and 2 blue spacers to it.
2. Attach to frame.
3. Add elastic as shown.

Step 5: Loading & Firing

To fire the catapult, you need one dark grey connector.

1. Hold catapult by the base as shown (the picture does not have elastics, but yours should).
2. Put ammo on as shown.
3. Hold ammo and pull back as far as the yellow rod.
4. Let go.
5. Watch a small piece of plastic fly across the room and hit your teacher in the face. Say someone else did it.

That's it!

Note: Step 5 is optional, but extremely funny.