The parent, who wants to remain anonymous, said his teenage child, a pupil at Peebles High School in the Borders, other pupils and teachers, were forced to “funnel” into the school’s main area to get out of the building, causing panic.
He says he and a number of other parents and pupils were concerned prior to the fire about the school’s “lockdown” policy.
He claims ground floor exit doors, but not classrooms, were kept locked as the head teacher feared a “Dunblane” type scenario.
Scottish Borders Council said a full debrief into the incident was under way.
Police Scotland said a boy has been charged with wilful fireraising and a report has been sent to the children’s reporter. A second boy was arrested and released, pending further inquiries.
The parent said: “My child is a credible witness. On her video, you can hear kids screaming.
“I strongly believe what I’ve been told, which is because the three doors were locked people couldn’t leave immediately and had to be funnelled out through another area.
“During this time there was chaos with children frightened and crying, hearing the sound of explosions before the roof came down.
“The pupils were then forced to scramble over fences to get off the grounds after first being placed in a hall right next to the burning building.
“Fires can kill people really quickly and the school was lucky no lives were lost.”
A spokesman for Scottish Borders Council said: “Thankfully, all pupils and staff were safely evacuated from the school last Thursday.
“A full debrief process is underway into all aspects of the fire and until this is completed it wouldn’t be appropriate to comment further on individual details. There is no ‘lockdown policy’ in place at the school.”
A spokeswoman for the Scottish Fire and Rescue Service (SFRS) said: “A joint SFRS and Police Scotland investigation into this incident presently remains ongoing.
“The SFRS was alerted to a fire at 12:57pm. The first appliance at the scene at 1:05pm was mobilised from Peebles Community Fire Station.”