It happened at Uniondale High School just before 3 p.m.
Police say a 15-year-old stabbed a 17-year-old several times in the neck, back and abdomen.
The suspect fled the scene and was later caught by Nassau County Police.
The victim was taken to the hospital where he is currently in stable condition.
Uniondale High School will be open on Tuesday, with social workers and psychologists on hand for students and staff.
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