A Malaysian teenage boy charged with the high-profile murder of a female schoolmate has been acquitted, with the judge finding the boy was mentally unsound during the incident.

He had fatally stabbed Yap Shing Xuen in a toilet at their secondary school on the outskirts of the capital Kuala Lumpur.

The killing of the 16-year-old girl had shocked Malaysia and sparked debate about the effects of social media on children.

Police previously had said they suspected social media could have influenced the boy, who was 14 at the time. The case was seen to have spurred authorities to limit children’s access to social media, which began earlier this year.

The boy – who cannot be named under Malaysian law as he is still a minor – had pleaded not guilty to the murder charge.

During the trial defence lawyers did not deny that the boy killed the girl, but argued that he had suffered from poor mental health for years and asked for his acquittal on the grounds of insanity.

An expert witness, a psychiatrist, testified that the boy had schizophrenia, and had suffered from mental illness and experienced delusions since he was nine years old.

The boy’s condition was said to have worsened during the pandemic and when he eventually returned to school. Lawyers had said that the boy would need medication for the rest of his life to manage symptoms.

On Monday the judge ruled that the boy would be acquitted, but also ordered for him to be detained and to receive treatment at a psychiatric hospital.

He will stay at the hospital until he is deemed fit to be released back to society, and would likely be subject to yearly assessments.

A defence lawyer told reporters on Monday that upon hearing the verdict, the boy appeared “neutral in emotion, but he understands what is going on, [that] it’s a very serious matter”.

The trial and verdict hearings were held in a closed court, which meant that the public and media could not attend.