"This is a major disaster. It is horrible, it is painful," the South American nation's President Irfaan Ali said Sunday night, CBS News reported.
The death toll had risen to 20 and several people were injured in the fire at the Mahdia Secondary School in central Guyana, the government statement said.
Ali said he ordered that arrangements be made in the two major hospitals in Guyana's capital of Georgetown "so that every single child who requires attention be given the best possible opportunity to get that attention."
Private and military planes have been sent to Mahdia, some 124 miles south of Georgetown, as the region is affected by heavy rains.
Guyana, a small English-speaking country of 800,000 people, is a former Dutch and British colony with the world's largest per capita oil reserves, which it hopes will help spur rapid development.
MNA/PR