The savage assault occurred at Narknarwaouptump School on Pattanakan 26 Road about 8:35am, as the students were dismissed from assembly, reports the Bangkok Post.
Media quoted teachers as saying the attacker was a Grade 8 school boy with special needs. Teachers and police from the Klongtan station were interrogating the boy.
The school is under the jurisdiction of the Bangkok Metropolitan Administration and remained open afterwards.
The young victim was rushed to hospital, where he died.
A woman security guard at the school said the young victim ran to her for help, holding his bleeding neck. He collapsed to the ground in front of her. She quickly pressed a cloth over his wound in hopes of stemming the blood flow.
The attacker ran up to her and stared down at the victim, she said. Teachers grabbed and held him.
She said the two boys were not friends. Just before the incident the attacker did not appear tense. He normally stayed happily with his friends, the guard said.
She said the school had a policy of searching for weapons, but it was not implemented regularly.