The performance divergence among 8th-grade students in the United States typically begins around the age of 13. This period marks the transition from middle school to high school, a time when academic rigor increases significantly. According to a study by the National Center for Education Statistics (NCES), 8th-grade students exhibit notable differences in achievement levels by the end of middle school, with those who have been in more challenging curricula showing higher proficiency in math and reading compared to their peers in less rigorous programs.

The reasons for this divergence are multifaceted. Firstly, the curriculum in 8th grade becomes more advanced, requiring students to master more complex concepts. This increased complexity can be challenging for students who have not been exposed to a rigorous academic environment. Secondly, the transition to high school often coincides with significant changes in students' personal lives, such as increased independence and social pressures, which can impact their academic performance. Additionally, the introduction of standardized testing in 8th grade, such as the National Assessment of Educational Progress (NAEP), can add stress and influence students' performance.

Data from the NAEP, which assesses students' proficiency in various subjects, reveals that the gap in achievement between students in higher-performing schools and those in lower-performing schools widens by the 8th grade. For instance, in 2019, the average scale score in reading for students in the highest-performing schools was 273, compared to 244 for students in the lowest-performing schools. This discrepancy highlights the importance of early interventions and the need for schools to provide a challenging and supportive environment for all students.