MOSCOW — Two masked teenagers armed with knives stormed a school in the central Russian city of Perm on Monday, wounding at least 12 people, most of them elementary school students.

A teacher and a 16-year-old pupil who tried to intervene sustained serious stab wounds to the neck, officials said.

The two youths — one said to be a current student, the other a former student who was expelled — entered School 127, in the Motovilikhinsky district around 10 a.m. They came in through a back entrance, unnoticed by a security guard, and broke into a fourth-grade classroom, local news outlets reported.

One assailant pounced on students while the other blocked the classroom door, a witness told the newspaper Komsomolskaya Pravda. “They said they’re going to slaughter everyone. We initially thought it was a joke,” the Perm news site 59.ru quoted another as saying.