Kyiv police reported bomb threats to the Ukrainian capital's schools on Friday as children returned to classrooms for a second academic year since Russia's all-out invasion.

Kyiv's police said its forces were checking educational facilities.

"We have received information about explosives in Kyiv's schools," police spokeswoman Yulia Girdvilis told AFP.

"All educational institutions are being checked by Kyiv police forces with the involvement of the State Emergency Service."

The police force also said any evacuations would be decided by schools and the police, calling on people to "stay calm."

Nearly four million students are returning to learning, both virtually and in the classroom, Ukraine's education ministry said.

Big cities and towns across Ukraine are regularly subjected to artillery, drone and missile attacks and thousands of schools have been damaged or destroyed, according to authorities.

"The nation preserved the opportunity for children to go to Ukrainian school," the head of Ukraine's presidential office, Andriy Yermak, said on social media. "Ukrainian teachers are real heroes."

More than 3.6 million children attend school in Ukraine, including nearly 900,000 who study remotely, said Yermak.

"The main thing is that our children will study," said General Valery Zaluzhny, commander-in-chief of the Ukrainian armed forces.

"And our educators, teachers and lecturers will teach. Because knowledge and culture are what distinguish us from the enemy."

Andriy Sadovy, the mayor of the western city of Lviv, said pupils will be learning to fly drones, releasing a picture of students behind computers.

"This is our new reality," he said on social media.

Kyiv launched its long-anticipated counteroffensive in June but has acknowledged tough battles as it struggles to break through heavily fortified Russian positions.