Detectives freed the teenager on Friday on police bail pending further inquiries. He had been treated in hospital on Thursday for non-life threatening injuries before being questioned.
Police say one male is currently in hospital in a critical condition in the Royal Victoria Hospital.
Paramedic crews and police raced to the scene on Thursday after the alarm was raised.
The stabbing is alleged to have happened in a toilet block and students said they noticed a “man lying on the ground” and a lot of blood around him.