Research has demonstrated that classification as an English learner (EL) is consequential for students, with important effects on educational achievement and attainment. Yet we know little about the mechanisms that underlie the effects of EL classification. This study explores whether, and to what extent, EL classification results in the treatments it is designed to trigger, and also whether, and to what extent, EL classification triggers unintended treatments. The study uses longitudinal school district data and regression discontinuity analyses to evaluate the treatment effects of EL classification. It examines two moments: initial classification as students enter kindergarten, and retained EL classification as students transition into middle school. Findings suggest that EL classification, for students near the margin of classification, results in many treatments it is intended to trigger including language assessments in early elementary, and English language development instruction and modified content instruction in middle school. In addition, findings show that EL classification triggers two unintended treatments among students; Classification limits access to English speaking peers in early elementary, and negatively impacts enrollment in a full academic course load in middle school.