Fair educational assessment in linguistically diverse contexts poses new challenges which call for the need to evaluate the impact of context-related language factors on student performance. Based on data from the Basque Autonomous Community in Spain, this research analyses the effect of different factors on a mathematics achievement test. Using generalised linear mixed models, the results explain differences in achievement among student groups based on home language, educational linguistic model, and test language through sociolinguistic status variables and correspondence between home language and test language. The paper highlights the importance of language-related factors and points out that achievement test scores in linguistically diverse contexts should be interpreted with caution to avoid unfair comparisons.