Used by a Sports Teacher to Get Pupils’ Attention

Puzzle 13 of Soft Things Piece of Cake Pack

Question is: Used by a Sports Teacher to Get Pupils’ Attention

and the answer is: WHISTLE

Word Meaning:

WHISTLE — A device designed to be placed in the mouth and blown, or driven by steam or some other mechanism, to make a whistling sound.

Definitions from Wiktionary.