IVANA SEKULARAC and ALEKSANDAR VASOVIC A boy gunned down fellow pupils in a Belgrade school in a pre-planned attack, shooting dead eight plus a security guard, officials said, prompting Serbia's president to announce tougher curbs on gun ownership. Using two handguns that belonged to his father, the 13-year-old fired first at the guard and three girls in a hallway and then shot his teacher and classmates in a history lesson, police said. The teacher and six pupils were hospitalised, some with life-threatening injuries. Veselin Milic, head of Belgrade police, said the attacker had two guns and two petrol bombs and…