OBJECTIVE: To analyze the prevalence of suicidal ideation and its associated factors in school adolescents. METHODS: Cross-sectional school-based study with 674 students from public and private schools in Teresina, Piauí, in 2016. Bivariate analysis was performed with the chi-square test and multiple analysis by the Poisson regression model to estimate prevalence ratios (PR) and 95% confidence intervals (95%CI). RESULTS: The study participants were mostly female (56.7%), black (77.4%), who lived with their parents (85%), whose mothers had schooling greater than or equal to 8 years of schooling (68.8%), with family income greater than a minimum wage (58.3%), practitioners of some religion (86.8%) and coming from public school (64.7%). The prevalence of suicidal ideation was 7.9%. Higher frequency of suicidal ideation was reported among female students (10.2%). Suicidal ideation was statistically associated with students who reported not living with their parents (adjusted PR = 2.27; 95%CI 1.26–4.10; p < 0.05) and those who reported having suffered sexual violence by other students, teachers or school staff (adjusted PR = 3.40; 95%CI 1.80–6.44; p < 0.05), among which the prevalence of suicidal ideation was more than three times that observed among those who did not mention this type of violence. CONCLUSION: The prevalence of suicidal ideation in school adolescents was associated with female students, who did not live with parents and have been victim of sexual violence at school. We recommend advising the school community and health professionals to identify signs of suicidal behavior, especially in those with suspicion or proof of the occurrence of sexual violence at school.