bang bang, you're dead, 50 bullets in your head
A threat. The number would rise to increase the severity of the threat. Claims of a million, or even infinity bullets, were not rare.
written by Ni*k Ke*t, approved by Log

If more information about the bullets seems appropriate, offer them this;
One's black,
one's blue,
one full of chicken poo."
written by excluded pupil, approved by Log

Not quite as effective without the preceding steps.
Make a "gate" with your hands, middle fingers touching. Say to your chosen victim, in a manner similar to that adopted when asking them to smell your cheese, "Open the gate!". Excited to see what will happen, your victim (the fool) will open the gate. Quickly, before they realise their mistake, make gun shapes with your hands, and shoot them down in a hail of "peeeoww"s and "er er er er er er"s. Repeat the rhyme over their twitching, bloody corpse.
written by Ni*k *unt, approved by Log

In addition to this, the composition of said bullets could be stated:
"One red, one blue, one made of doggie's poo".
written by Ma*t S*arp, approved by Susan

We were obviously less sophisticated at my school, where "opening the gate" would merely result in being punched in the face.
A triumph of substance over style.
written by He* Ph*ll*ps, approved by Mansh