Teenager Ty Townsley, from Tuffley, has died aged 15.
The Gloucester Academy pupil is believed to have died in his sleep on Saturday night.
Headteacher John Reilly paid tribute to the former Coney Hill rugby player.
He said: "He was a lovely lad, really popular.
"We really are in a state of shock and I am lost for words to be honest."
The school has set up a book of condolences for pupils to leave their messages for Ty, which will later be given to his family.
