Serbian 8th graders take exams in math, the Serbian language and a combined test in other subjects. The exam results are included in their overall rankings to get into high school, including selective schools.
But the mathematics exam on Tuesday was postponed until Thursday following revelations that this year's test had already appeared on social media.
Education Minister Mladen Sarcevic said police are investigating and those responsible will be punished. The leak has prompted calls for his resignation.
Hundreds of primary school graduates marched through Belgrade, gathering outside government headquarters and the Education Ministry, demanding that the exams be cancelled all together.
A similar test leak in 2013 forced the annulment of the exams.
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