Given the importance of relatedness for study motivation, motivation might be associated with faculty trust in students and, as such, with schools' student composition. Representative Flemish data of 5162 students and 1247 teachers in 57 Flemish secondary schools, gathered at the end of the 2013/2014 school year (April--May 2014), reveal that the association between faculty trust in students and students' autonomous motivation goes through students' trust in teachers. In schools with a higher socioeconomic student composition, students are less autonomously motivated, but this does not show because faculty trust is high, which generates more autonomous motivation--via students' trust in teachers.