We provide evidence from a randomized controlled trial on the effectiveness of a novel, 100-percent online math tutoring program, targeted at secondary school students from highly disadvantaged neighborhoods. The intensive, eight-week-long program was delivered by qualified math teachers in groups of two students during after-school hours. The intervention significantly increased standardized test scores (+0.26 SD) and end-of-year math grades (+0.48 SD), while reducing the probability of repeating the school year. The intervention also raised aspirations, as well as self-reported effort at school. [Additional funding for this report was provided by Esade, Fundaci'on COTEC, Fundaci'on CEOE, United Way Spain, Banco Santander, UNIR, Huawei, Fundaci'on Atlantic Copper, and Edvolution.]