Giovanni Vantaggiato, a 69-year-old husband and father of two, set off a bomb made from three gas canisters as pupils were entering the school in the southern city of Brindisi, killing 16-year-old Melissa Bassi and seriously wounding five or her classmates.
Prosecutor Cataldo Motta had accused Vantaggiato of "wanting to kill...and intimidate his country" and said the discovery by police of three other explosive devices following the school bombing proved he had "planned to carry out another attack".
Vantaggiato, who owns a fuel depot, had told investigators he was having financial difficulties after being swindled out of €400,000 euros and several of his clients - including the school - had cancelled their contracts with him.
He admitted to having made the bomb and tested the detonator the night before the attack, before installing the device in front of the Brindisi school and setting it off the next morning.
The attack - the first against a school in Italy - shocked the nation, sparking comparisons with a spate of bombings carried out in the 1970s by the militant left-wing Red Brigades.
The sentence comes just days after a school janitor in Sicily was arrested for murder after shooting a teacher five times.