The boy, who police say is a known youth gang member, was arrested in Frankston on Thursday, three days after a 14-year-old student from Glen Eira College was forced into a Volkswagen Tiguan in Glen Huntly.
The schoolboy suffered life-threatening injuries after coming out of the car while it was moving. He was taken to hospital, where he remains in a serious condition.
Inspector Scott Dwyer said police believe the Frankston South boy, who is being interviewed by police, was the main offender and flagged more arrests in coming hours.
"We have identified all those persons we believe responsible for that attack, and their arrests are imminent," he told reporters on Thursday.
About 10 minutes before the attack, two people armed with machetes robbed four teenagers of their mobile phones and other personal belongings.
Detectives are yet to confirm whether the armed robbery is linked, but believe the attackers were targeting students after school.
Extra patrols have been rolled out in the Glen Eira and Bayside areas, near where the incident happened.
Uniformed and mounted branch officers are focusing on areas around schools, but Insp Dwyer urged the public not to be alarmed.
"Just because you see police in key locations doesn't mean that that is a dangerous area," he said.
"We are there to ... deter crime as much as we are to investigate it."
Insp Dwyer said the number of Victorian youth offenders aged 10 to 17 is rising, with police monitoring 598 youth gang members across 44 gangs.
Of those, 444 have been arrested more than 1600 times during the past 12 months.
"That shows the level of commitment we have to targeting youth gangs," Insp Dwyer said.
The investigation remains ongoing, and anyone with information has been urged to contact police.
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