Serbia’s education minister submitted his resignation Sunday following two mass shootings, one of them at a primary school, that left 17 people dead, and the country’s government urged citizens to turn in all their unregistered weapons or run the risk of a prison sentence. Education Minister Branko Ruzic was the first Serbian official to resign over the shootings despite widespread calls for more senior officials to step down in the wake of the back-to-back bloodshed. Ruzic cited the “catastrophic tragedy that has engulfed our country” in explaining his decision. Weekend funerals were held for the nine victims of the shootings...