Purpose: Cannabis use among adolescents is a public health problem, one associated with intellectual and cognitive impairment. This study investigated the negative impact of cannabis use on adolescents’ school grades and intelligence. Methods: The study screened 1682 students during their university preadmission medical examination. Thirty-six students tested positive for cannabis, and 36 controls who tested negative for drugs were also included in the study. Sociodemographic and clinical data, school grades, and IQ (Wechsler Adult Intelligence Scale) scores were collected from cases and controls. Results: Cases were more likely to have lower final exam grades than controls in secondary and preparatory school, although they had higher grades than controls in primary school before starting cannabis use. Cases were more likely to have lower scores in verbal comprehension, perceptual reasoning, and total IQ than controls. However, the working memory and processing speed scores of both groups were equivalent. Students who started cannabis use at an earlier age (<15 y old) were more likely to have lower scores in total and all domains of IQ except verbal comprehension than students who started at a later age (>15 y old). There was an association between frequent use of cannabis and lower IQ scores (total and all domains). Conclusions: Cannabis use is associated with lower school grades and intelligence scores among adolescents, especially those who used cannabis frequently and at a young age.