Background. Young people constitute a large number of population worldwide, and majority of this population group lives in developing countries. They are at high risk of engaging in risky sexual behaviors. These risk sexual behaviors predispose youths to several sexual and reproductive health problems like STIs, HIV, unwanted pregnancy, and abortion. So, this study was conducted to assess the magnitude of risky sexual behaviors and associated factors among Jiga high school and preparatory school students, northwest Ethiopia. Methodology. Institutional based cross-sectional study design was conducted among Jiga town high school and preparatory school students. A total of 311 students were included in the study. Systematic random sampling method was used to select study participants. Data was entered using EpiData version 3.1 and it was exported to SPSS version 22 for further analysis. Descriptive analysis and bivariate and multivariate analysis were also calculated to determine factors associated with risky sexual behavior. Result. Forty-eight (16%) of respondents reported that they had sexual intercourse. From those who start sex, 44 (14.7%) were involved in risky sexual behavior which could predispose them to sexual and reproductive health problems. More than half, 27 (56.3%), of respondents first sexual intercourse was before their eighteenth birthday. The mean age and SD of fist sexual initiation were 17.2 years old and 1.35 years, respectively. Factors associated with risky sexual behavior include respondents between the ages of 20 and 23 (AOR: 5, 95%, CI: 1.59-15.98), drinking alcohol (AOR: 2.48, 95% CI: 1.13-5.41), and having poor knowledge towards HIV/AIDS (AOR: 4.53, 95%, CI: 2.06-9.94). Conclusion. A large number of in-school youths are involved in risky sexual behaviors like early sexual initiation, having multiple sexual partners, inconsistence use of condom, and having sex with high risk partner (CSWs). Age of respondents, alcohol drinking, and poor knowledge towards HIV/AIDS were factors associated with risky sexual behavior. School and community based programs in reducing substance abuse among youths and increasing their knowledge towards HIV/AIDS are important.