A Reuters correspondent on the scene in the Nerettes community near Port-au-Prince counted at least 10 people killed when the three-story La Promesse school building collapsed.
Students lay crushed under blocks of concrete while crying and screaming parents tried to find their children.
Police and troops from a U.N. peacekeeping force in the impoverished Caribbean country searched through the rubble for survivors.
"It's like an earthquake," said Brazilian Maj. Gen. Carlos dos Santos Cruz, the commander for U.N. troops in Haiti. "You have a lot of people under the debris. We don't know how many exactly but there are a lot of people there because they were attending school."
Police commissioner Carl Henry Boucher said more than 25 people had been taken to hospital in very serious condition.
He too said many students were still beneath the debris.
