Police are investigating Hong Kong International School after it expelled 11 students and disciplined others for drug use.
The school, in Tai Tam, did not inform police of its actions.
Last month administrators of its high school conducted an inquiry into violations of school policy about drug use.
It was initially thought just a few pupils were found selling and using marijuana. But as the inquiry progressed, the number of teenagers involved grew. HKIS would not confirm how many had been expelled but a source with knowledge of the situation put the figure at 11.
Police confirmed the school had not informed them about the drug offences, and they were investigating.
"Police have not received such a report from the school," a spokesman for the force said.
"Officers have already approached the school as part of their inquiries and will continue to follow up on the incident."
In a letter to parents last month, the school said it was not in a position to discuss the course of action taken against individuals. It said only that penalties had been applied, including suspensions and expulsions.
"We have conducted a thorough investigation in accordance with school policies and the law," the letter stated.
However, this runs contrary to the response given by police about the incident yesterday.
A police source went as far as saying it was the school's duty to report the matter to the force, and that failing to do so could have "major ramifications". The source said: "If they have kept the confiscated drugs they are in possession of evidence illegally. And if they have destroyed the drugs they have illegally destroyed evidence. It's that serious."
When the Sunday Morning Post asked HKIS for additional comment about the police inquiry, the school replied only with replicated quotes from the letter sent to parents.
School authorities said they had been rigorous in their investigation to ensure the most suitable course of action had been taken in each individual's case.
"We have been in communication with the parents of all students involved and we have done everything in our power to ensure this situation has been dealt with promptly and in an appropriate manner," it said in the statement.
The HKIS source said the incident began during a "Splash n' Dash" activity - a physical education exercise where students are required to run around Red Hill and swim at Turtle Cove Beach.
"A teacher found around 10 students under a bridge smoking marijuana. They were called into the office," the source said.
"There was a snowball effect, where over the course of three weeks, 46 students in high school were called into the office for interrogation and 11 were later expelled. It is still unclear how many were suspended."
The source said high school principal Patricia Klekamp called students into her office individually, collecting information from pupils' phones and asking them how many times they had smoked and with whom.
She then wrote down the names on the list and called in each of those pupils to interrogate them and to cross-check the names, the source added.