A Thai teenager killed his grandparents before opening fire at his school, where he shot five people dead and took his own life, police have said, in an attack that has shaken a country struggling to deal with gun violence.

The 14-year-old wounded more than 30 people in the ⁠shooting at the prestigious Debsirin Nonthaburi school in the Nonthaburi province on Bangkok’s outskirts. Nine people were in a critical condition, the Thai health department said.

For Prompong Namwong, an English teacher at the school, it was a normal morning until a panicked student from another class flung open the door, trying to find somewhere to hide. In the corridor, Namwong heard other teachers shouting that there had been a mass shooting downstairs.

When Namwong recounted the ordeal to the Guardian, he hesitated between calling the 14-year-old, whom he saw run in front of his classroom, “killer” or “student”.

At that moment, “I thought I was going to die, for sure,” he said.

The suspected attacker fired at least 26 bullets with a gun belonging to his grandfather, and was found with 34 additional ammunition rounds.

The suspected shooter remained inside the school after reports of the attack emerged, Arsit Sampantharat, the permanent secretary of the interior ministry, told PPTV.

A lockdown at the school was declared over in about half an hour. Namwong said that when he heard a knock, he thought it was the end of his life, but instead it was the police coming to their rescue.

Images from the scene showed students running with only socks on their feet as they were evacuated from the school.

The children, as is customary across Asia, had taken their shoes off outside their classrooms, but in the panic to escape the grounds, the shoes were left strewn around lunch tables and the stairs outside classrooms.

By the afternoon, the sports area where the students would usually have been playing was filled with officials and the media, as the country’s leader vowed to implement new gun laws.

The prime minister, Anutin Charnvirakul, told reporters the new law would allow only government officials on duty to carry guns, but did not provide any ⁠specifics, including whether the legislation would tighten gun ownership requirements.

Thailand has the highest rate of civilian gun ownership in south-east Asia, and in 2017 the Switzerland-based Small Arms Survey placed the country 13th worldwide for small-gun possession.

A number of mass shootings have occurred in recent years in Thailand, including incidents at schools and a large Bangkok mall, and a 2022 attack in the country’s north-east that ‌killed 37 people including 22 children.

The country’s licensing regime still lacks mental health screening for people applying to have a ⁠gun and does not require periodic reassessments ​for licence holders, according to Piyaporn Tunneekul, an ​associate professor at Nakhon Pathom Rajabhat University in Thailand.

Pichanan Thiengtham was in the teachers’ lounge when she heard the shots at about 10am. But she said she could not believe at first that it was a gun: “There were so many shots and they lasted for a long time without stopping.”

It was only when she saw one of the injured studentsthat she realised what was happening. “I took the injured student to get first aid and then transferred them to the first aid.”

Kiatikhun Verapongpradith, an emergency worker, described arriving as the shooting was taking place. He said his team had treated students with injuries to the back, chest and arms.

They found a male teacher lying dead on an upper floor of the school, and in another room they found a female teacher with wounds to her chest and arm, Verapongpradith said.

The school is part of the Debsirin network, founded by the Thai royal family. Its alumni include prime ministers, scholars and writers.

Namwong found out in the school’s group chat that some of his friends had not survived. “They’re all good teachers. I don’t know what to say.”

“My life has to go on, but right now … it’s a very great shock,” he said. “I never thought to myself that it would happen – really happen – in my school.”

Reuters and the Associated Press contributed to this report