The teen student who stabbed and killed another boy at school on Monday claimed the victim had been demanding money and assaulting him over the past two weeks, police say.

The stabbing occurred at Naknawaupatum School on Pattanakan 26 Road in Suan Luang district at about 8.35am on Monday when students were leaving morning assembly.

Police said that the 14-year-old attacker said the other boy had often extorted 20 baht from him at lunch breaks.

The alleged extortionist usually claimed he needed money to buy cigarettes, police said, and allegedly also punched the head and face of the other student.

"The boy had been treated this way three or four times over the past two weeks. Having the grudge, he bought a knife and took it to school this morning," a local policeman said.

Police had yet to verify the claim.

Police confirmed that the attacker appeared to be mentally stable and answered police questions clearly.

This contradicted the earlier speculation that the attacker might be a boy with special needs.

Police did not find any drugs in tests on the suspect.