Adolescents' school engagement is associated with high school completion; yet, few studies have examined interventions to improve school engagement for English Learners (ELs). In this mixed-methods study, we conducted a randomized controlled trial (RCT) to examine the effects of two years of the "Check & Connect" intervention on engagement outcomes (i.e., a self-report engagement questionnaire, attendance, disciplinary referrals, and course failures) of 358 high school ELs with reading comprehension difficulties. No significant differences were found between the treatment and comparison groups on a self-report questionnaire's subscales of behavioral disengagement (ES = -0.14), psychological engagement (ES = -0.22), academic and cognitive engagement (ES = -0.12), student-teacher relationships (ES = -0.10), and goal-setting and problem-solving (ES = -0.11), or on measures of attendance, disciplinary referrals, and core course failures. Then, we conducted qualitative interviews with a subset of the participants (n = 34) from the RCT to explore their perceptions of their engagement and the contextual factors affecting their engagement. Although interviewed participants reported being generally engaged in school, this finding was not corroborated by their interview responses nor other indicators of engagement collected during the efficacy study. Participants reported positive relationships with their "Check & Connect" mentors; however, these relationships were not sufficient to facilitate participants' academic, behavioral, or cognitive engagement. Implications for implementing "Check & Connect" with high school ELs with reading comprehension difficulties and measuring engagement with this population are discussed.