Pupils at Graham School were evacuated after staff received an email saying "a bomb would go off in the cafeteria."
Police were called to the school at 8.40am this morning (Monday May 15) following reports of a suspicious call to the school.
Pupils returned to classes after both the upper and lower school were evacuated earlier today.
In a text message to parents from Graham School, a spokesperson wrote: "This morning, emails were sent to staff at school to say a bomb would go off in the cafeteria.
"Both upper and lower site were evacuated for safety and police attended at both sites to undertake searches. They have given the all-clear for the return to the school buildings."
The hoax call was made on the first morning of this year's GCSE examinations.
A spokesperson for North Yorkshire County Council said: "Graham School had carried out emergency procedures as an immediate response to the suspicious call and the school was evacuated on both the lower and upper school sites.
"Students returned to their lessons after an hour once police established it was safe for students to go back onto the premises.
"Graham’s headteacher, Helen McEvoy, has praised students for their good behaviour and remaining calm during the evacuation."
A spokesperson for North Yorkshire Police said: "On attendance, it was established that the call was a hoax. The incident is currently being investigated by police."