"We are taking this extremely seriously," said Anna Westerholm, from the curriculum department of the Swedish National Agency for Education. "This undermines the entire purpose of the test and means that pupils are losing the opportunity to show what they know."
Several head teachers in Sweden have reported copies of questions from the Swedish and Swedish as a Foreign Language papers being shared on the platform, more than a day before pupils were due to sit the test on Friday.
Swedish elementary school pupils take the national test in year three, six and nine, making them about nine, twelve and fifteen years old respectively.
Their results affect entry to upper secondary education.
Westerholm said it was unclear how the questions had leaked, but speculated that some teachers may have left question sheets unguarded on their desks, allowing students to photograph them.
In February Uppdrag Granskning, an investigative programme on Swedish Television, reported widespread cheating in the test.