Police have warned of "an alarming increase" in drug dealers who sell to schoolchildren - in particular, those at English School Foundation (ESF) schools.
The trend has been growing over the last six months, police say, with three drug peddlers arrested.
A police source says the sale of cannabis is a lucrative trade for dealers and a way to make "easy money" but investigators do not suspect it is organised by triad gangs.
"From our investigations, it's clear that ESF schools have all got the problem, as their pupils generally have more disposable cash," the source said.
"There's been no link between the people we've arrested. These dealers will just buy the drugs themselves and sell it on for a profit."
"Chinese dealers, in particular, tend to handle transactions in their cars. Pupils get the dealer's phone number from friends, then arrange to meet at a certain place. The car will arrive, the pupil will get in and then they'll drive around the corner. Once the transaction is over, the pupil alights."
Those customers could be as young as 14 years old, the source added.
"We have arrested a few of these dealers but there's definitely been an alarming rise in this in the past six months," the source said.
"We're seeing a lot more action, especially that [modus operandi] of using a car. Dealers obviously think it's a safer way of doing a sale and not attracting attention."
The police claims are backed by observations by the Kely Support Group, a non-government-funded, fully bilingual youth charity that specialises in helping drug and alcohol addicts. Teenage drug use across the board was escalating, the group said.
"Based on our consultations and work in the community, there is wide speculation that hidden [or secret] drug abuse among the youth is on the increase in Hong Kong," Kely executive director Chung Tang said.
He also believed that the method of selling drugs to pupils in cars was not new in the city, and that from their experience the practice had been going on for the last few years.
An ESF spokeswoman said all its schools had a thorough drug policy in place, and that it was reviewed regularly. Drug education was part of the secondary school curriculum and police would be called if dealers tried to push drugs to pupils, she added.
Police made 5,371 arrests for drug offences last year. Of those detained, 913 were under 21 years old, official figures show.