The teenager's hand was cut after the incident in a class at Longhill High School in Rottingdean.
He was taken to hospital but wasn't thought to be seriously hurt and he was later discharged, police said.
Sussex Police said a 13-year-old boy was arrested.
He was released on bail as police continue to investigate.
A spokeswoman for the school, which has about 1,200 pupils, said a craft knife was used.
The stabbing comes about a week after teenager Luke Walmsley, 14, was stabbed to death at his school in North Somercotes, Lincolnshire.
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