Arsonists caused minor damage to Harlands Primary School in Haywards Heath by pushing burning items through its letterbox.
The attack on the school in Penlands Road happened between January 3 and January 5.
It is believed burning items, which were not identified, were put through the school's letterbox on Saturday night.
Headteacher Helen Thorne said: "I would suspect it was caused by bored teenagers who could have been using the porch as a shelter against the cold weather.
"It was potentially a very serious situation but luckily no major damage was done."
Anyone with information should call PC Ian Hancock on 0845 6070999.
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