Lol. office also got this kinda thing.
I rmb in primary school we had the pen mortar. Pilot G2 pen. Take out the ink tube then remove this plastic part and put onto of the spring. Push it down till u hear the pen click, then cut a small circular rubber ring from the pen rubber grip and stuff it into the mouth of the "mortar tube" to prevent the works from coming out when firing. There are a range of projectiles ranging from small cut pieces of eraser, stapler bullets, to 6mm bb pellets which is the best projectile.
For longer range u can stuff in 2 springs for more power.
Then fire at will at your teachers, they never knew who did it
Theres also the stapler bullet thing. That one damn pain kanna before in pri 5. We twist the 2 stapler bullets together to form a sort of barb wire and make lots of it and on days that have to wear PE shorts we toss them on the unsuspecting fellas chair. It will really prick your butt.