Eleven children and a teacher were wounded in Russia when a knife fight broke out at a school in Perm, a city 1,000km (620 miles) east of Moscow.
The victims were slashed when they tried to tackle two teenage boys who burst into a classroom wielding knives.
Two suspects, both 16, were detained by police and the incident is being treated as attempted murder.
Surgeons operated on 47-year-old teacher Natalya Shagunina and on four of the children, RIA Novosti news said.
The teacher remains critically ill, the others’ injuries are less severe.
Most of the injured children are aged 10-12 and have knife cuts to the head and neck.
The school was evacuated and will remain shut on Tuesday as police pursue a criminal investigation there.
Some witnesses quoted on social media contradicted reports from local security officials about what happened.
The witnesses said the two boys with knives had attacked the teacher and slashed pupils who rushed to defend her.
One, quoted by Komsomolskaya Pravda daily, said the two boys were clad in black, armed with hunting knives and acted together.
Earlier, a security source quoted by Interfax news agency said two boys had started a knife fight in a corridor, then burst into a classroom where younger children were studying.
Interfax quoted another local source as saying one of the boys had been expelled from the school, because of drugs and mental health issues, but got in after telling a guard outside that he was a pupil.
There are reports that one assailant, or both, had been angered by personal comments on social media.
Children said they saw blood on the floor as they were led out.