The incident allegedly occurred on the grounds of Indian Hollow School in Commack around 10:40 p.m. Tuesday.
Police say Clyde Ricks of Commack was arrested and charged with assault.
The two 19-year-old victims suffered multiple stab wounds. Both were listed in stable condition.
The 20-year-old suspect was awaiting arraignment on Wednesday. It wasn't immediately known if he had a lawyer.