This paper presents findings on the implementation and impacts of Leveled Literacy Intervention (LLI) in Oakland, California, where the school district conducted the nation's first randomized controlled trial of LLI in secondary grades. LLI is a short-term, intensive intervention designed to help teachers provide small-group instruction to struggling readers. Many school districts across the country have used LLI, which research evidence has shown to rapidly improve outcomes for students in early elementary grades. During the trial, secondary schools in Oakland faced various challenges implementing LLI, leading students to experience different levels of LLI duration, intensity, and fidelity. LLI had no impact on students' reading comprehension and a negative impact on their mastery of English language arts/literacy standards. Students who were pulled out of other classes to receive LLI were particularly negatively affected, possibly as a result of missing grade-level content. This study's findings highlight challenges in implementing effective literacy interventions for struggling adolescent readers.