Banksy surprised primary school children by painting a mural on the side of one of their classrooms during half-term.
The 14ft piece, showing a child with a stick chasing a burning tyre, was discovered at Bridge Farm Primary in Bristol at 6.55am on Monday.
Site manager Jason Brady found the artwork, which has been confirmed as genuine, with a letter in an envelope entitled "Dear Caretaker".
In the letter, Banksy wrote: "Dear Bridge Valley School, Thanks for your letter and naming a house after me.
"Please have a picture. If you don't like it feel free to add stuff, I'm sure the teachers won't mind.
"Remember - its always easier to get forgiveness than permission. Much love, Banksy."
Geoff Mason, who has been headmaster of the 550-pupil school for the past 15 years, said he was "thrilled" by the piece.
"I received a phone call this morning just before seven o'clock to inform me that we had a new addition to the school in the form of a piece of artwork by Banksy," Mr Mason said.
"I arrived to find this beautiful mural painted by Banksy. I think its fantastic that Banksy, who is so famous, has chosen to do this at our school.
"It was a complete surprise but I am thrilled."
Mr Mason said he had invited Banksy to the school, but was completely unaware of the artist's plans to visit over the weekend.
"The children wrote to Banksy explaining that we had changed house names and that we had done some designs," he added.
"We really just wrote to him just to let him know and to invite comment really, we certainly didn't expect something like this so it is fantastic.
"Banksy works in mysterious ways, it was an open invitation for him to get in touch with the school or come on site.
"It's brilliant for us and brilliant for the school community.
"It was a complete surprise this morning - I was in school last week catching up with a few things and had no inkling that this was going to happen."
The artwork appeared on the side of the reception year classroom, situated opposite the school's playing field.
Pupils at the school had previously taken part in a competition to name the school's houses, with the winning designs Brunel, Cabot, Blackbeard and Banksy.
Charlie Luka, seven, won the competition to name the house Banksy, and wrote to the elusive Bristol artist to inform him.
"When I came into school this art showed up on the wall and I was so surprised," Charlie said.
"It's here because I won Banksy for the house design. I chose Banksy because he is so creative.
"I was so shocked and amazed."
The artwork was discovered by Mr Brady as he walked through and checked the site before pupils arrived back after half-term.
"It took me by surprise, I was quite shocked," Mr Brady said.
"I was thinking 'is that real'? I like it, I do like it. I'm not a big person on art but I know of Banksy and I know of his work and he's from Bristol."
The school is planning to erect perspex over the piece to prevent it from being damaged, while security staff will also patrol the site.
Mr Mason added: "We are working with Bristol City Council, who own the building, as we would like to preserve and protect the artwork.
"Banksy is a world-renowned artist so obviously we would like to safeguard it and enjoy it over the years to come."
A spokeswoman for Banksy confirmed the artwork is genuine.