To better investigate why positive behaviour and misbehaviour occur, the study aimed to examine the relationships among teachers' autonomy support and students' advantageous comparison, non-responsibility, positive behaviour, and misbehaviour. We also examined the mediating roles of advantageous comparison and non-responsibility in these relations. The participants were 478 students with an average age of 14.6 ± 1.49 years, and the students included 259 males and 219 females. Structural equation modelling indicated that teachers' autonomy support had direct negative effects on advantageous comparison and non-responsibility. Furthermore, advantageous comparison and non-responsibility had direct positive effects on students' misbehaviour and had direct negative effects on positive behaviour. The relationships between teachers' autonomy support and students' misbehaviour and positive behaviour were mediated by advantageous comparison and non-responsibility. In line with previous work, teacher autonomy support might be critical to enhancing students' positive behaviour as well as reducing their misbehaviour. Since the mediating roles of advantageous comparison and non-responsibility were confirmed, we also suggest that strategies should be applied to eliminate students' moral disengagement.