Educators need ways to assess orthographic knowledge and differentiate word study instruction for secondary, emergent bilingual learners. In this study, the spelling of 199 students in grades 7-12 across eight features and four spelling stages was examined to understand students' orthographic development; all but two were learning Spanish and English. Spelling data were collected three times over a nine-month period. There were significant changes across the time periods that show students progressing across four stages of spelling development, with a large effect size (partial eta squared = 0.498). Data from a standardized assessment of language proficiency were examined to study the relationship between students' orthographic knowledge and their language scores. This study suggests ways to use spelling assessments to plan differentiated vocabulary and word study instruction.