The article examines the specifics of the development of imaginative thinking as a universal cognitive mechanism in middle school students (grades 7-8). The structure of this mental process, its distinction from verbal-logical and visual-figurative thinking, as well as the necessity of its targeted diagnostics for harmonizing intellectual development and successful mastery of the curriculum are analyzed. Based on empirical research (N=120) using a complex of psychodiagnostic methods (Raven's test, «Kohs Block Design Test», mental rotation test, «Pictogram» test), key age and gender features of imaginative thinking functioning in adolescents aged 13-14 are identified. The obtained data show positive dynamics in the development of spatial thinking by the 8th grade but simultaneously point to such typical difficulties as fragmented perception and rigidity of imagery. The results of the study have practical value for school psychologists and teachers, as the proposed diagnostic toolkit allows for identifying individual cognitive profiles of students and building targeted pedagogical strategies for developing the imaginative sphere in a comprehensive school setting. The conclusions drawn contribute to bridging the gap between the theoretical recognition of the importance of imaginative thinking and its real diagnostic practice.