Police said most of the students were out in the institution's recreation area when the collapse occurred.
About 100 students between five and 12 years-old attend the school.
The cause of the cave-in was not immediately known but the accident came a day after heavy rain fell across the region.
Haitian authorities have blamed shoddy construction for Friday's collapse of the three-story La Promesse school in Petionville, saying the building had little structural steel or cement holding its concrete blocks together.
Public safety officials said similar problems appeared to be behind Wednesday's collapse.
Eucher Luc Joseph, the secretary of state for public safety, said it was constructed "with practically no cement, no iron. It has been built in total violation of regulations".
Haiti's president recently warned that many buildings in the country may be at risk from poor construction and lax regulation.
United Nations security forces were assisting in the emergency operations, Sophie Boutaud de la Combe of the UN mission in Haiti (Minustah) said.
The UN mission is pushing for a wide spectrum of reforms in the country, including improved construction regulation.
"The ministry of education will do [an] assessment of all the schools to be sure that nonsecure [schools] will be closed," De la Combe said.
"That's the first step, and the second one will be to be sure that the new schools that will be built will be with codes of construction and respecting all the laws."