The purpose of this study was to understand the implementation and impacts of Leveled Literacy Intervention (LLI) in secondary grades. LLI is a short-term, intensive intervention system designed to help teachers provide small-group instruction to struggling readers. Many school districts across the country have used LLI, which has shown promise in improving outcomes for students in early elementary grades. This study used a randomized control trial in ten secondary schools in Oakland, California to study the program's implementation and assess its impacts on reading comprehension and general literacy achievement outcomes during one school year. Schools faced various challenges implementing LLI, leading students to experience different levels of LLI duration, intensity, and fidelity. In addition, LLI had no impact on students' reading comprehension and a negative impact on their mastery of English language arts/literacy standards. Students who received more LLI or 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 the importance of assessing whether the success of intervention programs in one context can be replicated with different populations and conditions.