Students attending the trial changed "We want justice" as an armored vehicle transporting suspect Giovanni Vantaggiato arrived at the courthouse, the Italian news agency ANSA reported.
Vantaggiato confessed to the May 19, 2012, bombing in which 16-year-old Melissa Bassi died and nine others were wounded at the trade school, named for the assassinated wife of anti-Mafia magistrate Giovanni Falcone.
Anti-mafia investigators were called in because of concern the bombing may have involved a domestic terrorism group or organized crime.
Vantaggiato, 69, hasn't offered a believable motive and has insisted he acted alone, ANSA said.
The bombing was close to the 20th anniversary of a mafia-ordered bombing that killed Falcone and his wife near Palermo.