The school was on the top floor of the three-storey building in Ita Faji on Lagos island, in Nigeria’s commercial capital.
Images from the scene showed rescuers searching through piles of broken concrete and twisted metal.
There was no word on casualties and the cause of the collapse was not clear.
Local media reported that the collapsed building was residential block containing a number of apartments.
Emergency teams have pulled injured people from the rubble. Some worried parents have arrived at the scene while others have gone to a local hospital to look for their children.
Ibrahim Farinloye, a spokesman for the National Emergency Management Agency, said the collapse happened at about 10:00 local time (09:00 GMT).
“It is believed that many people including children are currently trapped in the building,” he said.
It is not unusual for buildings to collapse in Nigeria. Materials are often sub-standard and the enforcement of regulations is lax.
In 2016, more than 100 people died when the roof of a church in Uyo, in the south of Nigeria, caved in.