Multilevel population-based cross-sectional study examining school substance-misuse policy and the use of cannabis, mephedrone and novel psychoactive substances among students aged 11-16 years in schools in Wales.

Objectives: To examine whether young peoples' risk of cannabis, mephedrone and novel psychoactive substances (NPS) use is associated with school substance-misuse policy.

Design: A cross-sectional survey of secondary school students combined with a School Environment Questionnaire and independently coded school substance-misuse policies (2015/6).

Setting: 66 secondary schools in Wales.