UPDATE 2.50pm: THIS morning's fire at Caldicot School was stopped in part thanks to the help of staff arriving for work at a nearby leisure centre.
Meanwhile Monmouthshire council said the arson attempt could have hit the education of hundreds of young people.
Station manager Matt Jones, who was the commander for the incident, said if staff at Caldicot Lesiure Centre hadn't raised the alarm the incident could have been much worse.
He said staff at the leisure centre had seen smoke coming up from the school as they arrived for work.
Mr Jones said someone had broken a ground floor office window, and that papers and curtains had been set fire to.
“It’s rather disappointing, for the want of a better word, that an individual could do this,” he said.
“Any school fire is a significant financial loss. It’s a terrible situation to be facing.”
A Monmouthshire council spokesman said: "We want to thank the fire service and leisure centre staff for their prompt action.
"Arson isn't a victimless crime and had this been successful it would have interrupted the education of hundreds of young people.
"We hope the police track the perpetrators soon, in the mean time we continue to plan to build a brand new school in Caldicot."
UPDATE 12.38pm: A FIRE at Caldicot School earlier today was a "clear act of arson", a fire officer has said.
Station manager Matt Jones, incident commander, said the swift action of attending fire crews extinguishing the fire prevented it from becoming a large incident.
He said: "This was a clear act of arson and the potential to have caused millions of pounds worth of damage was extremely close."
Several appliances had been mobilised to the incident, the fire service said.
If you can help the police or fire investigation into the incident call Gwent Police on 101 or Crimestoppers on 0800 555 111.
10.01am: A SCHOOL building in the Monmouthshire town of Caldicot was deliberately set on fire this morning, emergency services said.
Police and fire officers are investigating after the fire at Caldicot School, on Mill Lane, was put out at around 8.40am on Saturday.
No one was in the building and it is believed the damage was minor.
Police control room inspector Chris Haire told the Argus that officers were called at around 8.20am this morning to a smouldering fire in a school office.
A window had been smashed – Inspector Haire said something had been thrown through it, and that the fire may have possibly been started with matches.
He said there was very minor damage caused by the fire, but added it was a "deliberate ignition.”
Inspector Haire said the scene had been cordoned off and that both the fire service and the police were investigating – no arrests had been made this morning.
A worker in the fire service control room said that crews from Caldicot and Chepstow attended the fire, which was put out by 8.40am.
He said that no one was in the small ground floor office, which was attended to by around 16 firefighters.
If you can help police call 101 quoting log number 106.