The 'round object' exploded when the students of Karavaadi Municipal school at Vriddhachalam in the district touched it while cleaning the school complex, Inspector of Police S Rajendriran said.
Mahendra, Deva, and Thamizh Iniyavan, in the age group of 7-10, were injured in the explosion and admitted to government hospital at nearby Vriddhachalam, he said.
A case has been registered and a special team is probing the matter.
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