Adolescents living in vulnerable regions are more exposed to risk factors for drug use. The prevention of such use in school is a public policy that needs evaluation. Based on technical criteria and derived from a mixed research, this article analyses the quality of school-based prevention of drug use in Vitória, state of Espírito Santo, Brazil, and proposes improvements. A checklist of quality elements was completed with data from 16 projects proposed by 99 teachers from public schools. In 10 projects (62.5%), the approximate quality index was above 0.50. The majority of projects fulfilled the requirement of theoretical foundation (81.25%) and some of the methodological (93.75%), design (75%) and implementation (62%) requirements. Other requirements were absent: the majority were not designed by the whole school community (87.5%), and the participation of the family (62.5%) or the students as mediators (62.5%) was not considered. In general, contents of life skills (87.5%), positive relationships and alternative activities to drug use (56.25%) were not included. Activities for reinforcing the content were not described in any of the projects, and evaluation activities were described in only a few (31.25%). Many projects did not describe the inclusion of the project in the school curriculum (62.5%). Although, considering all items of effectiveness, regardless of their weight, more than half of the projects had an above average quality. The present items provide quality to the projects, whereas absent items indicate shortcomings to be improved using some of the measures described in this study.