The objective is to examine school-level program and policy characteristics and student-level behavioural characteristics associated with being overweight. Multilevel logistic regression analysis were used to examine the school- and student-level characteristics associated with the odds of a student being overweight among 1264 Grade 5-8 students attending 30 elementary schools in Ontario, Canada. Data were derived from the Physical Activity of Youth in Ontario Schools host study. Significant between-school random variation in overweight was identified [[sigma][subscript [mu]O][squared]=0:187 (0:084), P less than 0.001]; school-level differences accounted for 5.4% of the variability in the odds of a student being overweight versus a normal weight. A student attending a school that was in the action phase for the school-level construct "Availability and use of interschool programs" was significantly less likely to be overweight than a similar student attending a school that was in the initiation phase for this construct. Important student-level characteristics included physical activity (PA) and gender. Developing a better understanding of the school- and student-level characteristics associated with overweight among youth is critical for informing school-based prevention policies. Future research should evaluate if implementing and promoting interschool PA programming, and which types of interschool activities, are effective in preventing or reducing overweight.