Well this is a new good ol' prank! This is a perfect way to get other kids underwear & clothes splattered with any sauce (ketchup, mustard, taco, etc.)
First, you have to find you packets of any sauce like the ones you can get at fast food resturants as in ketchup or mustard which will be most commonly there (I use taco sauce since its the only thing in a packet in my school).
Second, go to the restroom and go to a toilet stall then lift the second seat up. You will notice on the bottom of the second seat (the poop seat) is two bumpy thingies to balance it. These are your friends since they pressure the packets to explode.
Third, fold the packets in half and have the opening ends, pace away from the toilet. And do not, I repeat, DO NOT open them! It will just make it harder to fold and get your hands messy and will not allow it to explode (and you'll be caught 'red-handed'). After folding them, put them under the bumpy thingies making sure they are perfectly alligned and that the seat is down.
Fourth, run out of the bathroom and just wait for the next victim to go inside, sit, and POP!