The negative consequences associated with out-of-school suspension (OSS) are widely recognized, yet its commonly utilized counterpart, in-school suspension (ISS), has received little attention. This study examined school and student characteristics that predicted ISS and its links to academic outcomes, using the nationally representative High School Longitudinal Study of 2009. The sample included 11,860 public high school students, equating to a nationally representative sample of 2,993,918 students upon the application of primary sampling weights for each student and balanced repeated replicate weights to account for students nested in schools. Students who were Black, male, of lower socioeconomic status (SES), or placed in special education were significantly more likely to receive ISS. Further, ISS was associated with lower grade point averages and increased likelihood of high school dropout. These findings raise caution about the use of ISS, particularly as schools consider using ISS as an alternative to OSS.