The van collided with the loaded dump-truck in the town of Nawabshah, around 270 kilometres (170 miles) north of Karachi in the southern province of Sindh.
The children, who were students of the Bright Future School from the nearby town of Daulatpur, had come to Nawabshah for a school trip.
Their van was on its way back to Daulatpur when the accident happened.
Due to the collision between the two vehicles, 18 students and three teachers were killed on the spot whereas 11 others sustained injuries.
The injured persons were shifted to People's Medical College Nawabshah for medical treatment.
Six of the injured persons were reported as critical who were subsequently shifted to the Intensive Care Unit (ICU).
“It was a head-on collision when the truck slammed into the school van on a single track dirt road. Apparently, speeding and recklessness resulted in the accident,” Abdul Rahim Gopang, a police officer, said by telephone from near the accident.
Television channels showed footage of parents running into a hospital in Nawabshah, frantically looking for their children, aged 10 to 16.
“All our dreams have been shattered. How can I live without my child,” a weeping Muhammad Akram, who lost his only son in the accident, said by telephone from the hospital.
Eleven children were injured and being treated in hospital, said Sindh's Information Minister, Sharjeel Inam Memon.
Appeals for blood donations for the injured were issued from the loudspeakers of mosques, local television channels said.
President Mamnoon Hussain expressed sorrow over the incident. He also expressed condolences to the parents of the deceased students.
ALLAH marney walon k luwahekeen ko sabro jameel ata farmaye..
beemaron ko jald sehat yaabi ata farmaye..Ameen..
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