The attackers pulled over and grabbed the teenage victim, who has not been named, at 5pm on Friday night.
They threatened to stab the sixth-form student – though he did not see any of them holding a knife.
Then they took him to a multi-story car park and stole his bank card, mobile phone and train pass.
A police spokesman said the car that pulled over in Bosville Road was ‘small and silver in colour’.
He added: “The victim is reported to have got into the car following threats from a passenger, before being driven to a car park in Bradbourne Park Road where cash, a bank card and a mobile phone were stolen. No weapon was seen during the incident.
“House-to-house enquiries and forensic tests have been carried out and an investigation is ongoing which also includes the analysis of any local CCTV opportunities.”
The headteacher of The Judd School, Jonathan Wood, told the Times: “We have certainly been talking to the student in question to establish the facts and support him. Our conclusion as a result is that there is not a threat to other students.”
Earlier that week a similar incident took place when a 16-year-old boy was held at knifepoint after boarding a train with three friends at Otford station.
He was travelling with three friends when they were accosted by four youths, and was forced to hand over his jacket.
Anyone with information about the Sevenoaks attack is asked to call 01622 604100, quoting reference 46/10525/2, or contact Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org