It may be three weeks until the cause of a fire at Nayland College on Friday night is known.
The blaze, which started at about 7pm in the school's horticultural shed, saw burning plastic cladding send up thick black smoke, prompting a number of calls to emergency services from hillside residents.
The shed and class sets of gardening tools were destroyed but the adjoining steel-clad automotive workshop was saved from significant damage.
The incident controller, Stoke senior station officer Tane Simpson, said a five-hour initial investigation on Saturday morning had been inconclusive, and samples would now be sent away for further examination.
It may take up to three weeks until the fire can be ruled as suspicious or accidental.
"We're just going to bide our time until the experts have had a good look and done their thing."
This was the time of year when suspicious fires were more common, with generally more "bored kids", he said.
The fire service was treating the incident as serious, Mr Simpson said.
The Stoke brigade had stayed at the scene until 10pm to make sure there were no flare-ups, and a scene guard was put on duty overnight.
Mr Simpson said because of the shed's distance from the main part of the school there had been a short delay in getting a mains water supply to the fire, with nine 30-metre lengths of hose used to span the distance from the nearest hydrant to the fire engine.
School principal Rex Smith said the school was being philosophical about the fire, and he was waiting to hear the outcome of the investigation.
"We're just hoping it's not arson," he said.
Horticulture was a popular subject at Nayland, with eight classes ranging from introductory to scholarship level.
But with only one week of term left, the fire would not be a major disruption.
Insurers would assess the damage next week and he hoped the unit would be back in action after the summer holidays.
"Hopefully it'll all happen quickly and if we can get some contractors in we'll get started on something."
The horticultural area is on a field below the main school buildings.
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