Effect of Full- Versus Half-Day Pre-K on Grade K–3 English Language Learner Designations

This study estimates the causal effect of randomized offers of full-day versus half-day pre-K on students’ likelihood of having English language learner (ELL) designations in early elementary grades. We leverage a randomized, controlled trial in a Colorado district serving primarily low-income Latinx families, where students assigned to full-day pre-K received more than twice as much instructional time. Although instruction was not formally multilingual, we hypothesize that additional English exposure in full-day classrooms may reduce the likelihood of a later ELL designation. Among students likely not fluent in English at pre-K entry, full-day pre-K offers reduce ELL designations in grades K–3 by 8–16 percentage points. These findings contribute to evidence on the long-term benefits of full-day pre-K and suggest the added costs of full- versus half-day pre-K may be offset by reduced need for ELL services in the early grades—an important consideration for district leaders weighing the value of expanding program duration.