Gender-based violence (GBV) is a global menace that has taken worrisome dimension in the Nigerian society especially among in-school adolescents. It is one of the challenges militating against total development of school aged children in Nigeria; hence, the purpose of the study was to investigate the form, causes and consequences of gender based violence among in-school adolescents in Ilorin metropolis in Nigeria. The study adopted a descriptive research of the survey type. There are 18 secondary school in Ilorin. Simple random sampling technique was used to select 10 schools from the available schools in Ilorin metropolis while random sampling technique was used to select 20 representatives from each selected secondary schools. A total of 200 representatives were used for the study. The result revealed a significant difference in the violence against male and female in-school adolescents. Females were shown to be more physically and sexually harassed as well as marginalized when compared to their male counterparts. The result would enable education stakeholders in Nigeria, especially classroom teachers, to be aware and alert on potential gender based violence against female students.