South Wales Fire and Rescue Service was called to St David's Junior and Infants School, Cwmbran, at about 20:10 BST on Saturday.
The fire started in an outdoor storage container and spread to a ground-floor classroom, causing minor damage.
Additional security has been put in place until the school reopens on Tuesday.

Officials said the link was not spotted until a post-mortem examination because the woman lacked some of the common symptoms of measles, such as a rash.
Measles is highly contagious, but it has been rarely fatal in the US.
The death comes after an outbreak of measles at California's Disneyland in 2014 that infected over 100 people.
Although no one died from that outbreak, it sparked an intense debate across the US about mandatory vaccinations for schoolchildren, which some parents oppose.
Officials said the case in Washington state was not linked to that Disneyland incident.
The woman was on medication that had weakened her immune system, Washington State Department of Health spokesman Donn Moyer told reporters.
She was hospitalised for several health conditions in the spring at a facility in Clallam County, Mr Moyer said.
"She was there at the same time as a person who later developed a rash and was contagious for measles," he added.
Officials have not revealed her name or her age but did say that she was not elderly. An autopsy concluded the cause of death was pneumonia due to measles.
"This tragic situation illustrates the importance of immunising as many people as possible to provide a high level of community protection against measles," Mr Moyer said.
There have been 176 cases of measles in the US so far this year, with 117 of those cases linked to the Disney outbreak, according to the US Centers for Disease Control and Prevention.
The last US death from measles was in 2003.

Analysts say the primaries are a good indicator of who may win the presidential elections later this year.
With 93% of the votes counted, governing coalition candidate Daniel Scioli had a 14-point lead over his conservative rival, Mauricio Macri.
The first round of the presidential election will be held on 25 October.
President Cristina Fernandez de Kirchner, who has been in power since December 2007, will not be able to stand again as the constitution limits presidents to two terms in office.
She was preceded in office by her late husband, Nestor Kirchner.
Their combined 12 years in office have been dubbed "the Kirchner era" and opposition candidates have been campaigning on a promise of change.
Mr Scioli, who is currently governor of Buenos Aires province, is the only candidate for the governing Front for Victory coalition.
His main rival is Buenos Aires mayor Mauricio Macri of the conservative Cambiemos (Let's Change) coalition.
With 93% of the votes counted, Mr Macri had won 24.4% compared to Mr Scioli's 38.3%.
Third was Sergio Massa, a congressman who broke away from the Front for Victory coalition to form his United for a New Alternative party.
Voting in the primaries is compulsory and is not restricted to party members but open to all those eligible to cast their ballot in the presidential polls.
Whoever wins is therefore seen as a favourite for the presidential polls on 25 October.
The primaries were introduced in 2009 to cut down on the number of candidates running in the presidential elections with contenders having to win a minimum of 1.5% to be eligible to stand in the first round on 25 October.
If no candidate wins 45% of the vote or 40% with a 10-point margin over the second-placed person, a second round will be held on 24 November.
Whoever wins will need to tackle a high inflation rate, which independent analysts estimate at 30%.
Mr Macri is campaigning on a promise to quickly free the markets from restraints introduced by President Fernandez while Mr Scioli advocates gradual change.
A suspicious fire which has damaged a classroom at a school in Torfaen is under investigation.
[NEXT_CONCEPT]
The death of a woman in Washington state in spring has been confirmed as the first US fatality from measles in 12 years, health authorities say.
[NEXT_CONCEPT]
The presidential candidate for Argentina's governing coalition has won a comfortable majority in primary elections held on Sunday.