More children were believed buried in the rubble and rescuers used bare hands and hand tools to get to them.
An Associated Press reporter saw the bodies of five children and two women, as well as dozens of injured students, some wiht bleeding heads, pulled from the collapsed building.
Matt Marek, head of programs for the American Red Cross in Haiti, said he thought the death toll will go much higher.
"This is going to be an all-day affair," he said.
Roughly 500 students from kindergarten through high school attend the school in the hills above Port-au-Prince, Petionville Mayor Claire Rudie Parent told the AP. She said she did not know how many were inside at the time of the late-morning collapse.
As rescuers carried the injured from the rubble of the concrete building, a swelling crowd erupted with wails and prayers.
"My child, my child," one mother yelled.
"There are no words for this," the mayor said as the hundreds of bystanders and rescue workers sesarched for survivors.
United National peacekeepers and Haitian police tried to clear a path through onlookers so that three battalions of military engineers from Brazil, Chile and Ecuador could assist in the rescue.
U.N. military commander Maj. Gen. Carolos Alberto Dos Santos Cruz said he had to park a kilometer (about a half-mile) away and walk to the wrecked school because he couldn't get by the crowd.
Parent said she suspected a structural defect caused the second story of the La Promesse school to collapse onto the first. The mayor said she doubted recent rains contributed to the collapse.
Volunteers arrived with shovels and axes and said they were going to try to deliver water to anybody who was trapped under the rubble. Other people shouted prayers as emergency vehicles raced up a winding hill to the school.
Haiti, the poorest country in the Western Hemisphere, has been struggling to recover from a chaotic year that has included widespread riots over rising food prices and a string of hurricanes and tropical storms that killed nearly 800 people.