Authorities said the gunman was a former student at the school and wore a black t-shirt featuring "Nazi symbols" as he carried out the attack, before killing himself.
ADVERTISEMENTAt least 17 people have died and 24 others have been injured after a shooting at a school in Russia.
It happened in Izhevsk, a city in the Udmurt Republic, 1,200 kilometres east of the capital Moscow.
"At the moment, investigators have established that 39 people were injured as a result of the crime," Russia's Investigative Committee said in a statement.
"Fifteen of them died, including 11 children and 4 adults. 24 victims were injured, including 22 children and 2 adults."
Authorities later confirmed that the death toll had been revised to 17 people, including six adults.
Russia's interior ministry, releasing a statement on Telegram, said that the gunman had committed suicide.
"Immediately after the police received a report of a shooting at School No. 88 in Izhevsk, officers from the Interior Ministry of the Udmurt Republic went to the scene of the incident and organised measures to apprehend the suspect," it read.
Authorities said the gunman had entered the school wearing a black t-shirt featuring "Nazi symbols" and a balaclava. The premises were evacuated.
He has been identified as 34-year-old Artem Kazantsev, a former student at the school who lived in Izhevsk.
"Investigators are currently conducting a search of his home to study the attacker's identity and motives," the Committee said, adding that an investigation was underway.
"His adherence to neo-fascist views and Nazi ideology is being checked."
The governor of Udmurt Republic, Alexander Brechalov, said on Telegram that a period of mourning for the victims had been declared until Thursday.
"A monstrous crime claimed the lives of very young children and adults," added Alexander Ivanovich Bastrykin, the Chairman of Russia's Investigative Committee. "This is a terrible tragedy, a heavy loss for all of us."
"The president deeply regrets the death of people and children in the school where an inhuman terrorist attack was carried out," Kremlin spokesman Dmitry Peskov told reporters on Monday.
A series of school shootings in Russia in recent years has prompted Vladimir Putin to tighten controls over gun ownership.
Last year, nine people, including seven students, were killed in a school shooting in Kazan.
A few months later, six people died and dozens more were injured in a shooting at Perm State University.
The deadliest school attack in Russia took place in 2004 in the city of Beslan when Islamic militants took more than 1,000 people hostage for several days. The siege ended in gunfire and explosions, leaving 334 dead, more than half of them children.
ADVERTISEMENTIn 2018, a teenager also killed 20 people at his vocational school in the Crimean city of Kerch before committing suicide.
Following the Kazan shooting, Putin also called on the Russian government to improve unified security and anti-terror protocols in schools.
Izhevsk, a city of nearly 650,000 people located just west of the Ural Mountains, is still home to factories that make Kalashnikov assault rifles.