A school pupil was taken to hospital after reportedly vaping an unknown substance.
Police have stepped up their presence at Ysgol Uwchwardd Bodedern following an incident on Thursday.
Officers spoke with pupils in order to gain any further information about what happened as part of their investigation.
Chief Inspector Jon Aspinall said: "Our priority in this case is to protect any further young people in the area from coming to harm."
"Incidents of this nature are rare, and we are taking this incredibly seriously to discover the circumstances around yesterday’s incident."
He added: "We have already begun working with partners to develop more education for parents and young people around this topic and will update on this soon."
"Our investigations are ongoing, and I would urge anyone with information about the incident to contact us as soon as possible."
Anyone with information should contact North Wales Police on 101 or via the website, quoting reference no. C161060.
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