The head of Hong Kong's only private school for young drug abusers said the numbers were very alarming because they pointed to a much wider problem.

'These are just known cases and many cases are still hidden in schools,' Christian Zheng Sheng College principal Chan Siu-cheuk said.

'There are probably five other students involved for every one student identified with drug abuse problems.

'If we do a projection, that means there would be nearly 2,000 students with drug abuse problems in schools in different districts in Hong Kong.'

Particularly worrying was the number of cases being found in areas that house some of the city's top schools, he said.

'About 130 cases were reported in the centre covering districts on Hong Kong Island and the outlying islands - the biggest number of all centres.

'I am worried that the drug problem has started invading Hong Kong Island, where there are many top band schools - which means not only bad students take drugs. Band 1 schools are troubled by this problem too.'

Mr Chan's comments were backed up by observations from the centre serving Kowloon City district, which has numerous top secondary schools and includes the wealthy neighbourhood of Kowloon Tong.

The Hong Kong Christian Service PS33 centre, which covers the area, says some young drug abusers from wealthy families spend thousands of dollars on cocaine.

'This is very much related to the family background. There are plenty of well-off families in the district and we found some students could afford HK$1,000 to HK$6,000 for a single intake of cocaine,' centre social worker Ho Fung-kuen said.

'They told us that HK$1,000 might be only enough for one tiny little amount of cocaine. So they sometimes had to spend more than that.'

Ms Ho noted that a school's banding was no longer a reliable indicator of whether it was likely to have a drug problem, with abusers increasingly being found in so-called elite schools.

'Of the 300 cases we have at hand, 60 of them involve students, some of whom are studying in Band 1 schools,' she said.

A few kilometres away, in Sham Shui Po, the difference lies in the amount spent and the type of drugs used.

'Young people in this district can only afford to spend a few hundred dollars at most. Some youngsters told us they spent about HK$100 on ketamine and it was enough for a few friends to share,' she said.

The younger age of drug abusers has contributed to another change in drug-taking patterns. Ms Ho said Yau Tsim Mong district, once a popular venue for youngsters' drug parties, had lost its appeal.

'The age of young drug abusers is getting lower. Some are too young to enter some entertainment premises. So most resort to taking drugs at home and at schools,' she said.

Ms Ho agreed that drug abuse was invading schools in nearly every district. At her centre alone, the number of new cases involving youngsters aged under 21 jumped by 60 per cent last year compared with 2007, she added.

Carol Ng Suet-kam, who is in charge of the Evergreen Lutheran Centre, said it had so many new cases that it had to draw up a waiting list.

'We used to receive about 12 to 15 requests a month for help from parents, teachers and social workers but the number has doubled since last November,' she said.

'Each new case has to wait for a month and a half before we can take it on board and there are about 30 cases on our waiting list at the moment.

'We only have about 10 social workers in our counselling centre and yet we have to deal with 300 cases a year.

'We had 140 new cases in 2008 alone.'

The Hong Kong Christian Service PS33 centre also has a waiting list, with young drug abusers waiting for a month or two before they can receive counselling.