noun
paper pellet; spitball
A small wad of paper, often chewed or folded, used as a projectile. The term is neutral but may evoke schoolyard mischief.
授業中に紙礫を飛ばして先生に叱られた。
I got scolded by the teacher for shooting a paper pellet during class.
彼はストローで紙つぶてを吹き飛ばした。
He blew a spitball through a straw.
礫 alone means a thrown stone or pebble; 紙礫 specifically refers to a paper projectile.
Compound of 紙 (kami, 'paper') and 礫 (tsubute, 'throwing stone'). The word literally means 'paper stone'.