Fire crews were called to Feltham Community College after reports of a blaze behind the main building this afternoon.
Three fire engines sped to the college campus in Browell's Lane and found a patch of grass alight on the fields backing on to the main building at 4pm.
It is believed the field was deliberately targeted by arsonists.
A spokesman from Feltham Fire Station said: "There was lots of smoke and as it was a school we thought it was best not to take any chances so sent three applicances.
"When we arrived we found a small patch of grass, about 100m sq alight. The fire was immediately extinguished.
"At this stage we can't comment on how the fire started."
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