More than 40 firefighters are still battling the blaze, which broke out shortly before 1pm this afternoon and are expected to stay on site "for some time". A roof is said to have collapsed at the school, where the art department and a sports hall are thought to have been severely damaged.
Unconfirmed reports claimed a clay oven in the art department exploded, starting the fire, which quickly spread throughout the building. Scottish Fire and Rescue sent extra fire appliances as the extent of the blaze became apparent.
The school is to close tomorrow and Monday while alternative arrangements are been made for pupils and staff.
One eyewitness said the fire had devastated entire buildings at the school. He said: "The art department and the old PE hall have all but gone."
Parent June Laurenson wrote on Twitter: "This is my daughter's school. Thankfully, all staff and students were evacuated and there appear to be no injuries. Sad for the art students who have lost their SQA portfolios, though."
Youngsters, who were seen fleeing the building, have been given shelter at Priorsford Primary School, where their parents were told they could collect them. There are no reported casualties. Students who live further away are being taken to a nearby church until the end of the school day, when they will be bussed home.
Tracey Logan, chief executive of Scottish Borders Council, said: “Peebles High School has been evacuated following a fire at around 1pm today. All pupils and staff are safe and accounted for. We are working closely with the Scottish Fire and Rescue Service during this ongoing incident.
“Pupils who live in the town are being taken to the nearby Priorsford Primary School. Pupils who use school transport are being taken to Peebles Parish Church and then on to their buses.”
She added: “We have taken the early decision that the school will be closed tomorrow (Friday) and obviously also the St Andrew’s Day holiday on Monday.
“During this time we will liaise with Scottish Fire and Rescue Service to ascertain the extent of the damage with a view to determining when the school can re-open and/or any alternative arrangements. Parents, pupils and staff will be informed through the normal channels.”
A message sent out to parents stated: “We are sorry to report that there is a fire at the high school. All children and staff have been evacuated and ALL ARE SAFE and are assembled as per a normal fire drill.
“Parents are asked not to come up to school by car as this may block access for fire vehicles.”
A Scottish Fire and Rescue spokeswoman said the fire was "severe".
“More than 40 firefighters continue to tackle a well-developed building fire at Peebles High School, on Springwood Road, Peebles. At present, we have nine fire appliances and two height appliances at the scene, working closely alongside emergency service partners.
“The alarm was first raised at 12.57pm on Thursday, November 28. The school was immediately evacuated, and there are no reported casualties."
She added: “Members of the public are asked to avoid the area at this time to allow access for the emergency services and we expect to remain in attendance for some time.”
Local MP Michelle Ballantyne said: "This is devastating news. I'm relieved to hear that so far there have been no casualties. My thoughts are with the students and staff at Peebles High School. This will be heartbreaking for the staff and pupils and over the next few days, I know council officials will be working very hard to ensure that pupils' education will not be disrupted."
Just an hour before the fire broke out, the school was promoting tickets for a production of The Addams Family, which was due to take place tonight.
The incident comes just months after Fife's Woodmill High School was severely damaged following a blaze which devastated huge swathes of the campus. Following the blaze, a 14 year old boy was charged with wilful fire raising.
John Stirling, a parent of pupils at Woodmill, said: "As a parent of Woodmill High pupils, this is upsetting to watch. Glad all Peebles High staff and pupils are safe and hope not too much damage has been sustained."