Three schoolboys aged 14 and 15 were arrested on suspicion of snorting ketamine yesterday after they fell ill in a park in Tin Shui Wai.
Police later arrested a schoolmate for allegedly selling them the drug.
The three boys, Form Two pupils at Pak Kau College, were found by a security guard in the park behind Yat Chi House, Tin Yat Estate, in Tin Sau Road, shortly before 11.30am.
The female guard told police one of the boys was unconscious on the ground and the two others were sitting dazed on a bench. Another boy passed out before paramedics arrived. All were in uniform.
They were taken to Tuen Mun Hospital, where the two who passed out regained consciousness. All were in stable condition last night.
A police source said: 'Preliminary investigation showed that the boys had become ill after taking suspected ketamine. White powder was found in their nostrils.' The source added that they must have taken a substantial dose to fall so ill.