Balalou said the head of one of the two exam centres at the Barthelemy Boganda high school had died “due to illness linked to the event”.
Panicked students and staff scrambled to escape, with some leaping from the school’s first floor, witnesses said.
Wounded students were rushed to hospital by ambulance, pickup truck and motorbike taxi, according to AFP reporters at the scene.
President Faustin Archange Touadera, attending a Gavi vaccine summit in Brussels, declared three days of national mourning.
The Central African Republic is among the poorest countries in the world. Since independence from France in 1960, it has endured a succession of coups, authoritarian rulers and civil wars.
The latest civil war started more than a decade ago.
The Government has secured the main cities and violence has subsided in recent years.
But fighting occasionally erupts in remote regions between rebels and the national army, which is backed by Wagner mercenaries and Rwandan troops.
– Agence France-Presse