Expressive writing (EW), or writing about one's thoughts and feelings, has been posited to reduce the working memory (WM) load that pressure and anxiety can impose on test-takers. The mechanisms of EW are far from clear, however, and social and developmental questions about its role in shaping children's engagement with academic contexts remain. We report a study with 250 10-12-year-olds (M[subscript age]=11.55; SD[subscript age]=0.48; 127 females), exploring gender differences in these younger children's use of EW before a high demand mathematics lesson on ratio. In contrast to the literature with adults, children assigned to EW had "greater" anxiety relative to control. The highest-achieving higher-WM girls also were most impacted by EW, showing decreases in immediate learning and retention relative to their counterparts who did not write expressively. A mediation analysis indicated that EW reduced learning gains by increasing children's anxiety during the lesson, thereby suggesting that EW functioned quite differently from its use in older youth and adults. These data suggest that emotion regulation skills may be an under-considered mechanism that underpins benefits of EW in older adolescents and adults. [This paper was published in "Contemporary Educational Psychology" v63 2020.]