Achievement goals are associated with varied students' outcomes, such as academic achievement and well-being. Exploring factors shaping adolescents' achievement goals, such as perceptions of significant others' goals, may benefit efforts to advance positive student outcomes. Prior research focuses on perceived teacher goals and considerably less on perceived peer goals. Even rarer are studies of variation in teachers' and peers' achievement goals over time. High-school students (N = 588; 53% female) completed questionnaires regarding their achievement goals and perceived teacher and peer goals, twice, six months apart. Cross-lagged analyses indicated that perceived teachers' mastery goals predicted increased student mastery goals and perceived teachers' performance goals predicted decreased student mastery goals. Perceived peer mastery goals predicted decreased performance goals and perceived peers' performance goals predicted increased students' performance goals. Half-longitudinal mediation analyses revealed that perceived peer mastery goals mediated the relationship between perceived peer performance goals and achievement goals. Results demonstrate that teacher and peer perceived goals predict changes in students' achievement goals differently. Awareness of class motivational dynamics and adaptation of classroom practice to encourage a group climate of peer mastery goals among students, alongside emphasizing mastery goals for individual students, will likely help promote beneficial achievement goals in students.