This study was conducted to estimate the impact, on health and behavioral outcomes, of replacing time screen-based sedentary behaviors with reading time in grades 5-12. A questionnaire was used to elicit students’ self-reports of their time spent on screen-based and non-screen sedentary behaviors and about their body weight, mental fitness, smoking, and experiences bullying (or being bullied) online. According to isotemporal substitution models, replacing one hour a day of television viewing with reading lowered the odds of being a current smoker, being overweight or obese, and being bullied or bullying others online. Replacing an hour a day of computer and video game use with reading had two of those positive effects. It reduced the odds of being a current smoker and of bullying others online (in each case, to a slightly smaller degree than when offline sedentary behavior was reduced). It also was associated with a 19% greater likelihood of high mental fitness.