"The compressed carbon dioxide extinguishers installed in Oslo schools between 2006 and 2011 may have a production fault … and can explode on contact," the municipality said in a statement.
"To be safe, the education administration … has decided to suspend and/or reorganise classes today," it added.
Around midday, it was unclear how many students had been sent home.
According to official municipal statistics, nearly 69,000 students attend Oslo's primary and secondary schools.
A valve production error was the cause of the urgent recall that began at midday and continued into the afternoon, when city officials held a press conference.
Newspaper Dagbladet wrote that the country’s preparedness directorate for natural disasters had warned of the explosion dangers as early as August 2011.
Kids in high school and middle school, or videregaaende and ungdomskole, were allowed to go home. In most schools, children were told to wait in supervised schoolyards until picked up by parents.
Some school principals chose instead to cordon off rooms where the suspicious canisters were hung and to continue classes.
“My immediate reaction is that someone has not done their job,” an angry parent of two was quoted by Dagbladet as saying.
The danger is said to lie in German-made extinguishers installed between 2006 and 2011. The potential risk was discovered on Wednesday, when it was learned the extinguishers had been recalled from the market.
City of Oslo education officials said Norske Sikkerhetspartner was the school system’s supplier for the extinguishers.
The purchasing and sale of fire extinguishers in Norway has become fraught with suspicion in recent years, after residents in the south of the country wanting to secure homes with extra devices were sold faulty or empty containers by men driving unmarked vans and posing as fire-safety officials.