Half the school starts creeping inside because of the shrill sound (7)

I believe the answer is:

screech

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'sound' is the definition.
(I know that screech is a type of noise)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for screech that I've seen before include "Loud shrill cry" , "Loud harsh cry" , "Squeal" , "Shriek" , "Cry raucously; type of owl" .)

I've seen this clue in the Sydney Morning Herald.

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