The language of schooling, a register closely linked to academic success, poses challenges for young pupils. These challenges are greater for immigrant and dialectal pupils who are expected to encode the register in question in a second language (L2) and a second dialect (D2) respectively. While the linguistic challenges facing immigrant and dialectal pupils have been extensively researched, those facing learners lying at the intersection of immigrant and dialectal pupils have not as yet received attention. The latter are immigrant pupils immersed in bidialectal communities, that is, communities where communication is performed through a standard and a non-standard variety of language. These pupils are confronted with the comparatively greater challenge of operating in the second dialect of a second language (L2:D2). Addressing a gap in research, this study sought to examine the school-based linguistic challenges facing L2:D2 learners, specifically as these manifest themselves in writing. To this end, a mixed-methods design was employed. One hundred immigrant and 76 non-immigrant pupils participated in the study, all in their final year of primary school. The latter served as a reference group. Data were collected mainly via writing and editing tasks. Interviews with pupils and teachers, together with a small questionnaire, provided supplementary information. Cyprus, a bidialectal country currently hosting an increasing immigrant population, provided the setting for this study. The texts produced by immigrant pupils contained unconventional forms and structures relating to register learning, L2 learning and D2 learning. Traces of L2:D2 learning were also detected. It was found that immigrant pupils - including the very early-arrived ones - underperformed in comparison with their Cypriot counterparts, not only in the language-specific aspects of the register (e.g. immigrants used more dialectal forms in their texts than Cypriots), but also in the non-language-specific ones (e.g. more immigrants than Cypriots assumed common ground with the reader). This performance gap cast light on a number of 'hidden' extralinguistic factors undermining immigrants' school-based written production: pupils' sociocultural circumstances, their 'outsiderness' in relation to the dominant community and the prevailing national ideology. These extralinguistic factors shift the attention away from L2 learning, a linguistic factor often portrayed as the primary source of immigrant pupils' language underperformance. This study contributed to the understanding of the construct 'immigrant pupil' by exposing its multifaceted and context-specific nature. Also, it illuminated an unexplored area, namely, the interplay between second language acquisition and second dialect acquisition, while informing educational policy and practice.