Fire crews were called to a serious fire at Buckholme Towers School in Lower Parkstone at 2.50 am on March 12.
The blaze that engulfed the first and second floors of the private preparatory school was not extinguished until the early hours of the morning and took more than 35 fire fighters.
Nobody was evacuated from their homes despite the blaze spreading to the roof space of the school.
Headmaster Iain Robertson said: “This is a very sad day in the history of Buckholme Towers School but we must not lose sight of the fact that this is just a building and that the children are the heart of the school.
Dorset Fire and Rescue Service commander praised the hard work of fire crews to put out the very well established fire and was thankful nobody was harmed in the blaze.
Mr Robertson also offered his thanks to the rescue services for their efforts in extinguishing the fire and assured parents the school was doing everything they could to ensure there is little disruption to their childrens education.
The cause of the fire is as yet unknown but under investigation, though not thought to be suspicious.
Police closed the A35 through Lower Parkstone until around 10am and gritters were called to avoid water run-off forming ice.