The director of the Clinical Center of Serbia, Milika Asanin, said that four people, including three students and a teacher, had been admitted to the facility’s emergency center after 9 a.m., and were being treated for gunshot wounds.

One student, around 13, had “life-threatening injuries” after being hit in the chest and head, Serbia’s health minister, Danica Grujicic, said at a news conference. Another student had been shot in her stomach and arms, according to Dr. Asanin. And the teacher, who was born in 1970, had also sustained gunshot wounds in her stomach and both hands.

“All patients are in the operating rooms, one student’s surgery is nearing the end and she is in a stable condition, we are waiting and monitoring the others,” Dr. Asanin said in an interview with Serbian state television.

Three official days of mourning, starting on May 7, were confirmed by Serbia’s minister of education, Branko Ruzic, in a televised briefing.

Video broadcast on local TV from the scene of the shooting showed dozens of vehicles, including ambulances and police cars, stationed outside the school, watched by local residents.