Title IX prohibits sex-based harassment (SBH) in all federally funded educational activities, yet little is known about SBH in K-12 schools. We used the Civil Rights Data Collection (CRDC) data (N = 96,440) to examine rates and school-, district-, and state-level predictors of SBH in K-12 schools. Results found the vast majority of schools had zero allegations of SBH. Girls were more likely to be reporters of, while boys were more likely to be disciplined for, SBH; Black students were disproportionately impacted by SBH. A series of negative binomial regression models showed that schools with higher student enrollment and rates of White students, greater numbers of supportive school personnel, Title IX policies, and state sex education laws were associated with increased allegations, reporting students, and disciplined students. State mandatory reporting for child abuse was associated with fewer reports; Title IX coordinators and security personnel were associated with more disciplined students. Our findings offer baseline information on SBH in K-12 schools and recommendations for future data collection, research, and policy priorities.