Background: Current guidelines suggest too little sleep, too little physical activity, and too much sedentary time are associated with poor health outcomes. These behaviours may also influence academic performance in school children. The primary purpose of this study was to examine the relationships between sleep, physical activity, or sedentary behaviours and academic performance in a school with a well-developed and integrated technology use and well-being program. Methods: This was a cross-sectional survey of students (n = 934, Grades 5-12) in an Australian school with a bring-yourown device (tablet or laptop computer) policy. Students reported sleep, physical activity, and sedentary (screen and nonscreen) behaviours. Academic performance was obtained from school records. Linear regressions were used to test the association between behaviours and academic performance outcomes. Results: Seventy-four percent of students met sleep guidelines (9 to 11 h for children 5-13 years and 8 to 10 h for 14-17 year olds), 21% met physical activity guidelines (60 min of moderate-to-vigorous physical activity every day), and 15% met screen time guidelines (no more than 2 h recreational screen time per day); only 2% met all three. There were no associations between meeting sleep guidelines and academic performance; however later weekend bedtimes were associated with poorer academic performance (− 3.4 points on the Average Academic Index, 95%CI: − 5.0, − 1.7, p < .001). There were no associations between meeting physical activity guidelines and academic performance. Meeting screen guidelines was associated with higher Average Academic Index (5.8, 95%CI: 3.6, 8.0, p < .001), Maths 7.9, 95%CI: 4.1, 11.6, p < .001) and English scores (3.8, 95%CI: 1.8, 5.8, p < .001) and higher time in sedentary behaviours was associated with poorer academic performance, including total sedentary behaviours in hrs/day (5.8 points on Average Academic Index, 95%CI: 3.6, 8.0, p < .001. Meeting at least two of the three behaviour guidelines was associated with better academic performance. Conclusions: Sleep and sedentary behaviours were linked to academic performance. School communities should emphasize comprehensive wellness strategies to address multiple behaviours to maximize student health and academic success.