Students' adaptive motivation to study tends to decrease over time. However, the reasons for this decline are not fully understood. Drawing on self-determination theory (SDT), we investigated whether changes in teachers' motivating style and students' associated need-based experiences could explain the motivational decline documented in the literature. A total of 472 Israeli seventh and eighth graders (in their first and second years of middle school) completed questionnaires at the beginning and end of the school year. Students reported their perceptions of their teachers' (de)motivating styles (i.e., autonomy support, structure, control, and chaos), the extent to which their psychological needs were satisfied or frustrated, and their motivation to study. There was a significant decrease from the beginning to the end of the school year in 7th- and 8th-grade students' perceptions of autonomy support and structure provided by their teachers, students' autonomous motivation, and their experienced need satisfaction. There was a significant increase from the beginning to the end of the school year in 7th and 8th graders' perception of their teacher as chaotic and the students' experience of need frustration, controlled motivation, and amotivation. A growth curve multilevel model (GCMLM) indicated that the perceived changes in teachers' motivating and demotivating styles, together with the changes in the students' reported need-based experiences from the beginning to the end of the year, could account for these changes in students' motivation. Teachers should develop and maintain a need-nurturing environment to prevent a drop in student motivation.