Fire damage to a school gym thought to have been started deliberately, has been estimated at £100,000.
About 19 firefighters attended the blaze at Cwrt Sart school
The blaze started at Cwrt Sart Comprehensive School, Briton Ferry, near Neath, just before 2200 GMT on Sunday.
Police said three people were spotted near the building - separate from other buildings on the school site - and are appealing for information.
Officers have been carrying out house-to-house inquiries.
Acting Chief Inspector, Simon Beltcher, said: "At approximately 10pm we had a phone call from a member of the public that a building in the school was on fire.
"The fire service attended and a gymnasium's been set alight and there's a considerable amount of damage to this gymnasium.
"We strongly believe it is deliberate at this stage and are making inquiries to try and find those who caused the damage.
"This act has affected children in the area. It appears to have destroyed their gymnasium at the school and we would ask that if anyone has information, please contact Neath police station."
Three fire engines and 19 fire officers were sent to tackle the blaze.
Officers have appealed for anyone who saw anything suspicious in the school grounds between 2130 GMT and 2215 GMT to contact them.
Detective Sergeant Mark Lewis said: "We have evidence to suggest that at least three males were in the school grounds near to the gymnasium between these times.
"One of these males was wearing a white or light-coloured hooded top which had its hood pulled over a light-coloured peaked cap. The other males were wearing darker clothing.
"We are obviously keen to trace these three individuals at least to eliminate them from our inquiries.
"I have no doubt that someone knows the identity of the culprits and would ask that the local community supports us in investigating this crime which is obviously going to have a massive impact on the pupils and staff at Cwrt Sart School."
Anyone with information can contact police on 01792 456999 or Crimestoppers anonymously on 0800 555 111.