Seek help from somebody you trust. It can be a teacher, a doctor, a fellow parent or an NGO. You must keep it in mind that in theory if you seek help from somebody in an official position (eg. teacher), they have an obligation to report it as using or carrying drugs under 18 is a criminal offence, but they are not likely to do so. Most doctors refuse reporting on the basis of patient confidentiality. Private people and NGOs have no obligation to report.