The animal had come out of Upper Dihing Reserve Forest at around 11.30 AM and entered the premises of a nearby school where it seriously injured the 17-year-old student, forest department sources said.
Angry over the attack, locals armed with sharp weapons clobbered the animal to death, the sources said.
Later, the carcass of the leopard was taken away by the forest department and the boy admitted to hospital.
Environmentalists claimed that increasing human-animal conflict was due to encroachment of forest area by humans. This had reduced the habitat of animals and caused leopards to leave the forests in search of food.