Language for academic purposes is an important concept, not always recognised and developed within the education system. The ability to use language for learning can be difficult for individuals who are educated in a second language. They are required to master complex concepts in a language they are still acquiring. We aimed to discover how secondary school learners performed on an aspect of academic language: complex sentence comprehension. A group of 464 adolescent participants' performance on the grammatical understanding subtest of the Test of Adolescent Language was analysed in relation to their status as first or second language English learners, their grade, gender, literacy experiences, preferred modality of learning, and other factors deemed to influence language acquisition in modern society. The results indicated that the majority of learners achieved within the average range. There were significant differences between the male and female participants and the junior phase ESL males achieved the lowest scores. It was concluded that it may take 8-9 years of formal schooling for some individuals to acquire the requisite academic language proficiency, particularly if they are learning in their second language. This has important implications for the teaching and assessment of second language learners.