Three measuring instruments are used in this study of 12- and 15-year-old children's passive vocabulary, active vocabulary and lexical diversity. Children are drawn from samples delimited by two social class backgrounds and by West Indian (Jamaican) background. Research was performed in north London and South Yorkshire. Highly significant differences are found in lexical access, by age, to semantic fields representative of knowledge categories of the school curriculum and highly significant differences by ethnic background at 15 years. In lexical diversity, with controls applied as in this study, there is no significant difference between sample means. In active use of specialist words of Graeco-Latin origin there are highly significant differences between the social classes and the social groups at 15 years, the age when final assessments of potential in education are being made. Because Graeco-Latin words populate the knowledge categories of the school curriculum almost to the exclusion of Anglo-Saxon words, the writer proposes that a ‘lexical bar’ is at work for some social dialect users in societies which use English as the principal language of communication. This bar has the effect of hindering the users of some social dialects from a ready access to the lexis of semantic categories essential for success in education.