According to the United Nations Human Development Report (2016), poor, marginalized, and vulnerable groups still face substantial barriers to access post-secondary education and employment. These marginalized and vulnerable groups include women, girls, indigenous peoples, ethnic minorities, people with disabilities, migrants and refugees, the LGBTI community, and those discriminated because of their socio-economic status (UN Human Development Report (2016). Increasingly, proficiency in mathematics is required for careers and for solving daily problems in life (Starkey & Kline, 2008; Ramaa, 2015). Basic numerical, mathematical, and scientific skills are an important mechanism to further education and to enable individuals to improve their job market potential. The risk of underachievement in mathematics around the world is greatest for students from low-income backgrounds, as well as linguistic and ethnic minorities (Ramaa, 2015; National Research Council, 1989). In India, these factors are compounded by lack of early exposure to math, poor teacher quality, and large class sizes (ASER, 2014). Although some research has explored difficulties in arithmetic for economically disadvantaged elementary students in India (see Ramaa, 2015), the challenges that marginalized adolescents encounter in completing secondary mathematics courses has been largely unexamined. In this qualitative exploratory study, conducted from October 2015 to February 2016, we sought to understand the characteristics of adolescents from socio-economically marginalized communities in Mumbai and Bangalore, India, and their determination and perseverance to overcome challenges in mathematics and complete their secondary education. In particular, the study followed students who had dropped out of mainstream school and enrolled in India's National Institute of Open Schooling Program to complete their high school education and attempt the 10th standard board exams. Results from data collected in the academic year showed that there were multiple factors, including lack of learning foundational skills at the primary school level and the type of intervention provided to complete high school, which influenced the students' achievement in mathematics. Findings from the study inform policy and programmatic decisions for students enrolled in India's National Institute of Open Schooling Program.