With a focus on within-person effects, this study investigated mutualism among academic skills (reading, math, science) and between those skills and verbal working memory in a general population sample and groups with high or low skills from Grades 2 to 5 (2010-2016, N = 859-9040, age 6.27-13.13 years, 49% female, ethnically diverse). Mutualism was found between reading and science in all high-ability groups, and between reading/math and verbal working memory only in high-math students. These results remained the same when controlled for socioeconomic status and gender, and with sensitivity analyses. High-skill students (especially high-math students) may improve academic performance through accumulation of academic knowledge and mutualism between academic and cognition. Such mutualism may be driven by high-quality, intensive academic practice.