A meta-frontier approach is used to evaluate the efficiency with which inputs into the education process are converted into outputs, using data for secondary schools in Wales obtained from the 2015 PISA study. The efficiency of schools in which teaching is conducted through the medium of English is compared with that of Welsh medium schools. On most measures, inputs into the latter are relatively high, having an impact on their observed levels of efficiency. Improving PISA scores in Wales likely requires enhancement of the performance of Welsh medium schools, but there needs to be explicit recognition of a trade-off between such improvement and the realisation of policy goals that are broader than those evaluated in PISA.