Serbia's education minister has come under fire after math tests were leaked for 8th grade graduation exams.

Serbian school children take the exams in mathematics, Serbian language and a combined test in other subjects at the end of primary school. 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 tests already appeared on social media.

Education Minister Mladen Sarcevic says police are investigating and those responsible will be punished. The incident has prompted calls for his resignation.

Several hundred primary school graduates gathered outside the Education Ministry building Tuesday demanding that the exams be cancelled.

A similar test leak in 2013 forced the annulment of the exams.