The government's brought in new style league tables to help parents choose schools. But do we really know what makes a good school? And how far can schools really transform lives? Researchers have long believed in a so-called 'school effect' that counters, at least in part, factors such as social and family background. But how easy is it to measure this kind of effect, and can parents really be given a clear guide as to which school is best for their child? Or has too much emphasis on factors such as social background made schools complacent about what they can achieve? Fran Abrams talks to head teachers, educational experts, the schools minister and the new head of Ofsted as she investigates what difference schools can really make.