Preval said that structures throughout Haiti are also at risk of collapse due to poor construction and insufficent governance.
"It is not just schools, it is where people live, it is churches," he said at the rescue scene.
Rescue workers, assisted by the International Red Cross, the Haitian Red Cross, and members of the UN Haiti peacekeeping force, used tools and their bare hands to dig through to children buried in the debris of the school in the hills above the capital.
"My son who is 15-years old, he's dead. He's my only son," said 40-year-old Josiane Dandin. "I don't know what I'm going to do."
"My son who is 15-years old, he's dead ... I don't know what I'm going to do"
Josiane Dandin, parent of a collapse victim
Another woman screamed for her missing 12-year-old daughter. "I don't know if she is dead or alive," she said.
One boy whose legs were trapped by debris begged rescuers to cut his feet off, a firefighter said.
The aid group Doctors Without Borders said it had taken 85 injured adults and children from the school, half of them with life-threatening injuries.
France expressed its "deep emotion" for the victims and promised aid.
"At the request of the Haitian authorities, France is preparing to send as soon as possible a civil security team to help the Haitian authorities in rescuing the victims who are still buried," Bernard Kouchner, the French foreign minister, said in a statement.
A new storey had been under construction atop the three-floor school when it fell in, also destroying or damaging five homes next to it.
"This construction did not meet normal standards," Senator Yvon Bissereth told the AFP news agency.
"We are going to ask the minister of education to make an inspection of all the schools built in the same way," he said.
About 500 students from primary through to high school ages attend the school in Petionville, but officials said they were unsure how many were inside at the time of the collapse.
The impoverished Caribbean nation, which lacks sophisticated rescue equipment, is continuing to struggle with the destruction wrought by four tropical storms and hurricanes that hit in quick succession this year, killing more than 800 people and destroying 60 per cent of the crop harvest.