Background: The youngest children in an academic year are reported to be educationally disadvantaged and overrepresented in referrals to clinical services. In this study we investigate for the first time whether these disadvantages are indicative of a mismatch between language competence at school entry and the academic demands of the classroom. Methods: We recruited a population sample of 7,267 children aged 4 years 9 months to 5 years 10 months attending state-maintained reception classrooms in Surrey, England. Teacher ratings on the Children's Communication Checklist-Short (CCC-S), a measure of language competence, the Strengths and Difficulties Questionnaire-Total Difficulties Score (SDQ), a measure of behavioural problems, and the Early Years Foundation Stage Profile (EYFSP), a measure of academic attainment, were obtained at the end of the reception year. Results: The youngest children were rated by teachers as having more language deficits, behaviour problems, and poorer academic progress at the end of the school year. Language deficits were highly associated with behaviour problems; adjusted odds ratio 8.70, 95% CI [7.25-10.45]. Only 4.8% of children with teacher-rated language deficits and 1.3% of those with co-occurring language and behaviour difficulties obtained a "Good Level of Development" on the EYFSP. While age predicted unique variance in academic attainment (1%), language competence was the largest associate of academic achievement (19%). Conclusion: The youngest children starting school have relatively immature language and behaviour skills and many are not yet ready to meet the academic and social demands of the classroom. At a population level, developing oral language skills and/or ensuring academic targets reflect developmental capacity could substantially reduce the numbers of children requiring specialist clinical services in later years.