The public should avoid speculating that bullying had triggered the shooting at Debsirin Nonthaburi School in Nonthaburi's Bang Kruai district, says Prime Minister Anutin Charnvirakul.

Investigators should first establish the facts. He also called for tighter gun controls, warning that people carrying firearms in public without being authorised officials could face serious criminal charges.

He rejected suggestions that teachers should carry guns for self-defence, saying they were not law-enforcement officers.

Mr Anutin also warned that gun owners could face prosecution if weapons under their control were stolen and used in a crime.

A 14-year-old student on Friday used his grandfather's 9mm pistol to shoot teachers and students inside the school after shooting dead his grandparents at home. He also took his own life, leaving nine people dead and 21 injured, including 19 students and two teachers.

A friend, identified only as M, said he had known the boy from Matthayom 1 until Matthayom 3 (Grade 7-9) and was certain his friend had never been bullied, teased or assaulted by classmates. He said the teenager rarely spoke to others and usually sat at the far-left corner at the back of the classroom.

The boy had not been angry with or involved in arguments with any classmates. He performed well academically. However, he repeatedly complained about his Thai-language teacher, saying he was unable to cope with the amount of homework and had been given a failing grade of zero in the subject.

The boy had tried several times to make up the grade, eventually succeeding, and had frequently discussed the issue with M.

M also said the teenager had previously brought a BB gun to school in his bag, showing it to friends and using it to play-shoot on school grounds. After another student reported the incident, a teacher confiscated the gun and deducted points from his conduct record under school rules.

On the day of the shooting, M said all students, including the suspect, attended the morning flag-raising ceremony and classes as normal. The shooting took place during the second lesson, mathematics, while the teacher was explaining homework and preparing students for revision. "Before the shooting, there were no warning signs or any tense atmosphere," M said.

The sister of the boy's grandmother insisted he was a good child who had never caused trouble for his grandparents or anyone else.

He was not easily influenced by friends, went to school in the morning and returned home on time, and spent weekends at home rather than visiting others. She said he played computer games, although she did not know which games.

She said he was not violent, describing him as a well-behaved child who kept to himself and did not get involved with others.

She also said his grandparents, who were among those killed in the shooting, had never pressured him.

Police said they had interviewed more than 20 teachers and students and will speak with the Thai-language teacher mentioned in connection with the case. Police are also examining where the live ammunition came from and whether the suspect had previously received firearms training.