ABSTRACT After World War II, socialist states developed a new schooling system aimed at building an egalitarian society. While there is a solid body of research discussing the relationship between the intent and effect of the egalitarian design of socialist school systems, the importance of school maturity assessments for the socialist project has received only minor attention. This article discusses how the introduction of school maturity assessments developed by experts in medicine, psychology, and pedagogy in 1960s Hungary and East Germany contributed to the visibility of children who were deemed immature. It shows that both socialist states developed institutional solutions beyond the standard school system for tackling the problem of school immaturity in children; these solutions, in turn, evoked different reactions from parents. As the article discusses, parents used various avenues to oppose the official expert assessments with which they disagreed. In both countries, parents increasingly voiced their opinions in complaint letters addressed to the state in a quest to overcome what they perceived as a severe threat to their children’s ‘normal’ development. By tracing parental bottom-up initiatives, the article investigates how parental agency ultimately shaped state policies and expertise despite the asymmetrical power relations that were present in these authoritarian societies. To show the interplay of parental agency, experts, and the state, we employ a combination of two methodological approaches, the sociology of expertise and the concept of Eigensinn, for understanding the spaces of negotiation and the role expertise plays in it. This article thereby sheds light on parental agency and its impact on expertise and state policies concerning school maturity in two socialist states that offered very different institutional answers to the problem; bureaucratic approaches towards complaint letters also varied significantly.