Introduction Although a few research have tried to explore the relationship between problematic smartphone use (PSU) and school engagement, most of them are limited to relatively simple correlation, and the mechanism needs to be further explored. This research focused on the relationship between PSU and school engagement/disengagement, and intended to verify two mediation paths. Methods We conducted two studies in 2019 at a middle school in China. 289 students (44.6% girls), aged 11–18 (Mage = 13.25, standard deviation [SD] = 1.73), participated in Study 1, a longitudinal cross‐lag analysis intend to verify the relationship between PSU and school engagement/disengagement. Using a separate sample, Study 2 explored the mediating roles of academic procrastination and sleep quality. Four hundred thirty‐two students aged 11–19 (42.1% girls, Mage = 16.11, SD = 1.56) participated in this cross‐sectional study. In both studies, all participants completed self‐report measures in classrooms during regular school hours. Results In Study 1, the results showed that PSU (T1) could significantly predict school engagement/disengagement (T2), but school engagement/disengagement (T1) could not predict PSU (T2). In Study 2, we found that academic procrastination could mediate the effect of PSU on school engagement, and sleep quality could mediate the effect of PSU on both school engagement and disengagement. Conclusions Results highlighted that the school engagement/disengagement of adolescents can be influenced by PSU through several different ways, through which we can protect adolescents from the negative effects of PSU.