The mishap occurred this afternoon when the boy, identified as Vachan, had accompanied his mother Vijaya to the school where she had joined as teacher only two days back, they said.
"It seems when Vijaya was busy with other students in the class, and the boy went outside. Later she came to know that her son has gone missing. After a search, Vachan's body was found inside the sump. He was rushed to a nearby hospital where the doctors declared him brought dead," Vanasthalipuram police station inspector P Gopala Krishna Murthy said.
"Based on a complaint lodged by the boy's father Anil Kumar, we are in the process of booking a case of criminal negligence under relevant sections of IPC," he said.