A boy with a “blank expression” stabbed and slashed 21 pupils and a security guard with two kitchen knives at his high school before being tackled by an assistant principal.

A 16-year-old student wielding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school early yesterday, injuring 21 people, about half of them seriously, officials said.

Four students are believed to have been seriously injured at a high school near Pittsburgh, where a student with a knife stabbed or slashed others – injuring as many as 20.

Media in the US are reporting that a number of people have been stabbed outside Franklin Regional High School in Murrysville in Pennsylvania.