The 17-year-old was found at City College in Cumberland St at 2.45pm on Saturday.
Police initially treated the death as unexplained.
However, they confirmed on Sunday morning that there were no suspicious circumstances.
As the matter had been referred to the Coroner, police could not comment further.
The 17-year-old's name had not yet been released.
"Police's thoughts are with the man's family at this time,'' a spokesperson said.