Although his identity has yet to be officially confirmed, he was named in the messages as Jack Bostock.
The teenager suffered serious head injuries after an incident outside the pupil referral unit on Daventry Road at lunchtime on Wednesday.
By the time paramedics arrived he was in cardiac arrest, and he died later at the Bristol Royal Infirmary.
It is still not known what he died of as a post-mortem examination had yet to be carried out last night.
Neighbours said they saw an ambulance at the scene and a teenage boy sat on the ground surrounded by friends.
By yesterday friends and family had left more than 20 bunches of flowers on the pavement outside The Park, and balloons and photographs of Jack were attached to the gates.
School friends stopped in the rain to read the tributes, gathering to share their grief and hugging each other.
Among the notes attached to the flowers was one from Jack's close family.
It read: "Jack, our wonderful son and brother. It hurts so much that you have gone, our hearts are breaking.
"We love you so much Jack, and will miss you forever.
"Our memories of you Jack will never fade. Forever in our hearts Jack, my love, love Mum, Dad, Sam, Rosie, Ethan and Tom."
A message of condolence from The Park's education team was attached to a large floral tribute, and Jack's classmates left their own messages.
One tribute read: "RIP Jack, from all the post 16, RIP m8. Sleep tight, god bless, see you on the other side."
Another said: "Jack, RIP, best wishes for you in heaven, from James Mac xxx.", while a third note in the flowers read: "Jack, RIP mate, I will always remember our times together. RIP babe, love u, Jane xxx."
Attached to the gate and laminated to protect it from the weather was a condolence card from two friends.
The message read: "To Jack Bostock, every breath and step we take we will always think of you.
"It all happened so quick, we done everything we could to help you and it just didn't work.
"You will never be forgotten. All our love, Chris, Grant xxx."
Jack went to The Park's pupil referral unit, run by the council's education department on the site of the former Merrywood School, which helps youngsters who have problems at mainstream schools.
The police have confirmed the incident is not being treated as suspicious, and Avon Coroner's Office has been notified.
Officials are waiting for a post-mortem to be carried out before formally opening the inquest.
The health and safety executive has also been made aware of the incident, and Bristol City Council is carrying out its own internal investigation.
You can leave tributes to Jack Bostock at Lasting Tribute.