During the secondary school stage, students’ motivation to study may decrease and affect their future expectations, which are exclusively directed toward the search for employment, with the consequent abandonment of academic training. The main objective of the present paper was to examine the sources of motivation to study and the future expectations of secondary school students, as well as to develop a predictive model of their future expectations based on the variables studied. The sample consisted of a total of 35,943 students from different Spanish high schools, with an average age of 15.83 (SD = 0.28). The instrument used was the placement tests referring to the PISA 2018 report. On the one hand, the results showed that the main source of motivation for secondary school students to study responds to some kind of imposition either from the surrounding environment or internally, which appears to be represented by identified or controlled extrinsic motivation. In terms of future expectations, important factors included the fundamentally expression of their intention to continue studying rather than to stop studying, facts or economic support which they considered as influential to their capacity to study, and the opinions of others such as parents and friends. On the other hand, sex showed some significant differences in terms of future expectations but did not predict them. The regression model explained 20.9% of the variability of future expectations based on variables such as grade repetition, reasons that discourage studying (not being interested in the contents and never studying), and the influences on future expectations (school grades and subject mastery). Finally, the structural equation model revealed that grade repetition predicts the reasons that discourage studying and these in turn impact future expectations which are influenced by school grades, performing well in a specialty, and having talent. Likewise, there was a negative correlation between repeating a course and school grades, performing well in a specialty, and having talent. Based on these results, it would be advisable to improve the intrinsic motivation of secondary school students by means of educational actions that contribute to the adjustment of their future expectations and attend to the students’ own interests, desires, and competencies, all with the main purpose of contributing to meaningful learning and facilitating professional orientation, and above all, attending to diversity to reduce school failure.