Support from parents and school staff are important for adolescent well-being. However, few studies have examined the role of parental involvement and adult social support at school on high school students' mental health, specifically, suicidal thoughts and behaviors (STBs). Data were collected from 362,980 high school students (51.85% female) from 412 schools in Georgia. Students reported perceptions of parental involvement, adult social support at school, experiences of face-to-face victimization, cyber victimization, and STBs. Hierarchical linear modeling (HLM) results showed that perceived parental involvement and adult social support at the student and school levels were negatively related to adolescent STBs. Face-to-face victimization and cyber victimization at the student and school levels were significant predictors for STBs. Student-level perceived parental involvement and adult social support mitigated the relationship between peer victimization and STBs. However, school-level perceived adult social support exacerbated the relationship between face-to-face and cyber victimization and STBs. It is essential to attend to the differential effect of student-level and school-level adult social support on youth mental health. It is also important to promote parental involvement and adult social support to promote mental health among high school students.