The phenomenon of low-cost private schools "mushrooming" in poor areas of sub-Saharan Africa and South Asia, and elsewhere, is now well-documented. Findings from research by the author's teams and others show that these schools are serving a majority (urban and peri-urban) or significant minority (rural) of the poor, including significant proportions of the poorest of the poor. Concerns are raised in the literature about their implications for social justice. In "The Idea of Justice," Amartya Sen asks us to rethink ideas about justice; instead of the quest for a Rawlsian "transcendental institutionalism", he argues for a comparative approach, grounded in the practicalities of human behaviour. Linking Sen's ideas on justice with the grassroots privatisation leads to the tentative conclusion that those concerned with promoting social justice could agree to help improve access to, and quality in, the low-cost private school sector, rather than focus on the public education sector. Paradoxically, this could be true even for those whose ideal is an egalitarian public education system.