A very small number of young people enter youth custody between age 16 and 18 (about 4 in 1000 males), yet the consequences are severe. They spend an average of 7 months in youth custody and such incarceration has been related to negative outcomes in the longer term even if they can establish themselves in the labour market. In this paper, we evaluate whether there is a relationship between GCSE qualifications in English and maths and the probability of youth custody using administrative data in England. We are hindered in this because the majority of young people who end up in youth custody are not entered or fail their GCSEs in these subjects. Although regression results are consistent with educational achievement being a factor in why people end up in youth custody, they strongly suggest that both non-entry/low achievement and youth custody are correlated with severe vulnerabilities which are partially picked up by the explanatory variables available in administrative data (in particular indicators for special needs, disadvantage and being from some ethnic minority groups). Another interesting insight is that for many, problems only emerge (or at least become evident) in early or middle adolescence. [This paper forms part of a larger project funded by the Nuffield Foundation "Youth custody: Educational influences and labour market consequences."]