Police were called to the school at 8.40am this morning (Monday May 15) following reports of a suspicious call to the school.
Pupils have now returned to classes after both the upper and lower school were evacuated.
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."
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."
The hoax call was made on the first morning of this year's GCSE examinations.