In many cognitive developmental studies, young children ( < 6 years) fail to understand that changing the appearance of a person, object, or animal does not change its underlying reality. They appear to believe that a cat wearing a dog mask is genuinely a dog (appearance/reality distinction) and that a boy wearing a dress is genuinely a girl (sex/gender constancy). These skills may be affected by various influences: testing methods, training on the constancy of biological traits, child’s or sibling’s diagnosis of gender dysphoria, and child’s diagnosis of autism. This study aimed to partially replicate the study of Arthur et al. (Journal of Experimental Child Psychology, 2009, Vol. 104, pp. 427–446) showing that experimental lessons emphasizing the constancy or otherwise of biological traits affected appearance/reality and sex/gender constancy performance. The study examined the influence of school lessons with content on sex/gender stereotyping and pro/anti constancy on the performance of young English children (mean age = 5;6 [years;months]; N = 58) on appearance/reality and sex/gender constancy tasks. Children performed above chance on sex/gender stability (change over time) but performed below chance on sex/gender constancy and appearance/reality tasks (change due to superficial alterations). These scores are comparable to those in nearly all previous studies. Children’s scores were not influenced by school lesson content, although not all schools provided content. Conclusions are drawn about the effect of lesson content, which may be too diffuse and long term to affect performance. The level of children’s performance and what this means in absolute terms about children’s understanding of sex/gender stereotypes and the possibility of a child changing sex/gender is also discussed.