The explosion happened on the morning of September 5, when students at the Yen Phu Primary School in Thanh Hoa Province attended a ceremony to celebrate the new school year.
After the ceremony ended, teachers tried to cut the wire to release a bundle of 20 balloons up into the sky. As parents and students were rushing to get some of the balloons, a series of explosions occurred.
It was identified later that a teacher standing nearby had accidentally touched the balloons, which were filled with hydrogen, with a burning cigarette, causing them to explode.
The explosion caused 10 students to suffer burns on their arms and faces.
By Monday, nine had been discharged from the province's children hospital and one had returned to school.
The teacher who caused the accident also suffered a burn on his arm.
The fine was made following a government decree on smoking in public places, said Principal Trinh Huu Tung.
The teacher has been working at the school for 30 years.
Doctors said it is very dangerous to fill balloons with hydrogen as the gas has very small fast-moving molecules which can easily diffuse through the balloon's surface and a balloon full of hydrogen can easily explode if it is placed near hot air or even a light bulb.