Volleys of gunfire erupted at the compound shortly before two dozen policemen rushed out of the gates clutching children in their arms and ending a siege.
The armed men stormed Siem Reap International School earlier on Thursday, demanding money, weapons and a vehicle before police ended the standoff, with witnesses reporting several shots fired.
The hostage-takers killed the boy when the authorities declined to meet all of their demands, and police then raided the building, Cambodian Information Minister Khieu Kanharith said, quoting the deputy national police chief, Neth Savoeun.
"They also threatened to kill the children one by one. Then our forces decided to storm the school," Kanharith said.
"They also threatened to kill the children one by one. Then our forces decided to storm the school"
Cambodian information minister
A Canadian embassy official confirmed a Canadian child had died, but gave no further details.
Cambodian police chief General Hok Lundy said the child was a boy aged about five.
"All of a sudden, the gates opened and people came out with children in their arms," John McDermott, a freelance photographer, told CNN. He said ambulances were at the scene.
"It seems everything is under control now," he added.
The hostage crisis unfolded at Cambodia's tourism hub of
Siem Reap, near its famed Angkor Wat temple complex and home to many expatriates.
Reports said up to 15 nationalities were represented among the hostages.
The attackers' motives were not immediately clear.
The men, believed to be armed with a single AK-47, had demanded six more of the machine guns, six grenades, $1000 and a van to take them to Thailand, a witness said by telephone.
Witnesses said several dozen soldiers and policemen had sealed off the area and surrounded the school. One witness added that children in a second building entered by the kidnappers had left unharmed, but was unsure whether they were released intentionally or escaped.
A suspect lies on the ground
outside Siem Reap School
Approximately 70 students attended the school located about one kilometre outside Siem Reap on the road that leads to the international airport.
Hostage-taking, normally for money, is not uncommon in the war-scarred Southeast Asian nation, which is awash with weapons left over from decades of civil war, including the Khmer Rouge genocide of the 1970s.