Classrooms and chairs were scarce when the school term began this Monday.
Half a million children - nearly double last year's enrolment - went to school as President Pierre Nkurunziza fulfilled a pledge, made when he was sworn in, to scrap school fees.
"We registered 264,853 pupils last year," said Reverien Gahungu, director general of primary education. "Now we have an extra 234,000."
The abolition of fees has made school a possibility for the estimated 4 million people in this central African nation who get by on less than 55p a day - about half the total population. The government will lose a mere pound;821,000 from waiving the fees, but now needs pound;16m to build new classrooms and provide other facilities. Saidi Kibeya, the education minister, is appealing to private aid and development groups to build mobile schools.
Two weeks ago, the president met church representatives and asked them to lend equipment to help schools cope with the overwhelming number of students who registered for free education.
Burundi is struggling to recover from an 11-year civil war, which began in 1993, between the former army, dominated by the Tutsi ethnic minority, and rebels from the Hutu majority.
President Nkurunziza, a former rebel leader, was elected after a four-year transitional government was set up to put an end to the war.