In order for schools to develop systematic and realistic strategies for extending children's linguistic repertoires, it is imperative that teachers and allied professionals have access to scientifically informed accounts of the variable but structured nature of the everyday speech used by children. Because there is insufficient information addressing grammatical variability in school children's speech, it is easy for teachers to misinterpret normal social patterns of variation as the product of error or confusion. This article addresses the lacuna in our understanding of grammatical variation in childhood by presenting a case study of variable subject-verb agreement in the speech of children aged between seven and 11. A detailed quantitative analysis of the co-variation between non-standard and standard variants in children's discourse reveals a heterogeneous, but intricately patterned, system. Furthermore, socially motivated patterns of variation remain stable across the age range examined and are unaffected by increasing exposure to formal education. The tenacity of vernacular norms raises a number of important issues pertinent to the teaching and learning of standard spoken English, including the extent to which children can be expected to substitute standard variants for non-standard ones in spontaneous discourse. (Contains 4 figures, 10 notes, and 2 tables.)