In this prospective study, we recruited a sample of Belgian adolescents (N = 886) to investigate to what extent perceived teachers' motivating style relates to quality of motivation in the beginning of the school year and, in turn, changes in study effort and procrastination by the end of the school year. After controlling for initial levels of study effort and procrastination and for a shared variance due to classroom membership, we found, through path analysis, perceived autonomy support and structure to relate positively to autonomous motivation, which in turn predicted increased study effort and decreased procrastination at the end of the school year. The findings are discussed from a theoretical and practical standpoint. Most educators would agree that some adolescents may postpone their assigned school work and avoid putting forth effort when doing their homework. Apparently, students' motivation to do their homework can largely explain whether they may behave in this way. While some students may study because they find the learning content enjoyable, interesting, and valuable (i.e. they are autonomously motivated), others may feel rather compelled to do so (i.e. they are controlled motivated). Research conducted under the framework of Self-Determination Theory (SDT; Deci & Ryan, 2000; Ryan & Deci, 2017) has indicated that the more students are autonomously motivated, the more they thrive, as indexed by greater engagement and effort-expenditure, better learning, and higher achievement (e.g. Guay, Ratelle, & Chanal, 2008). It is therefore important to investigate likely antecedents of students' motivation. Towards this end, SDT-based research has shown that students benefit more when they perceive the learning environment as autonomy-supportive (i.e. when teachers consider students' interests and perspective) and well-structured (i.e. when there are clear, consistent, and properly justified guidelines and rules) (