This study evaluates the impact of a program leveraging large language models for virtual tutoring in secondary education in Nigeria. Using a randomized controlled trial, the program deployed Microsoft Copilot (powered by GPT-4) to support first-year senior secondary students in English language learning over six weeks. The intervention demonstrated a significant improvement of 0.31 standard deviation on an assessment that included English topics aligned with the Nigerian curriculum, knowledge of artificial intelligence and digital skills. The effect on English, the main outcome of interest, was of 0.23 standard deviations. Cost-effectiveness analysis revealed substantial learning gains, equating to 1.5 to 2 years of 'business-as-usual' schooling, situating the intervention among some of the most cost-effective programs to improve learning outcomes. An analysis of heterogeneous effects shows that while the program benefits students across the baseline ability distribution, the largest effects are for female students, and those with higher initial academic performance. The findings highlight that artificial intelligence-powered tutoring, when designed and used properly, can have transformative impacts in the education sector in low-resource settings.