A 12-year-old girl wounded in a school shooting in Thailand died on Saturday, the health ministry said, bringing the death toll to eight, including the teen assailant's grandparents killed in their home.

On Friday, 14-year-old boy shot his grandparents before going on a killing spree at his school north of Bangkok and ultimately turning the gun on himself, according to police.

The health ministry said Saturday that the 12-year-old girl, who had been treated at a nearby hospital, had died.

"Another person has passed away... a 12-year-old girl," the ministry said in a statement.

The other victims included a maths teacher, a psychology teacher and other teaching staff, according to police.

More than 30 people were injured in the shooting, Prime Minister Anutin Charnvirakul said during a visit to the school.

Fourteen people remained in hospital as of Saturday evening, seven of them in critical condition.

Police are questioning more witnesses as part of their investigation, including the suspect's parents.

Thailand has one of the highest rates of gun ownership in the region, with around 10 million firearms estimated to be in circulation -- roughly one for every seven inhabitants.

Previous promises to tighten gun laws have failed to prevent repeated shootings.

In February, Thai police shot and arrested a teenager who opened fire at a school in southern Thailand, killing the principal and wounding two students.

The teenager had used a police officer's weapon in the shooting.

In 2022, an ex-policeman armed with a gun and a knife stormed a nursery in northern Thailand and killed 24 children and 12 adults, in one of the country's deadliest massacres.