
Schools who achieve badly one time will get less popular and then less money, so they cannot afford to improve their standards next year, increasing their unpopularity leading to a 'spiral of decline' where working-class students get a below-par education. Middle-class students get better education.
Popular schools get oversubscribed and therefore can choose their pupils- and will recruit 'ideal' high-achieving middle-class students to ensure their success. Schools attract parents through advertisement of facilities and high achievement- schools are given an incentive to increase their standards of teaching, which in turn benefits the children in them.
Parental choice is valued and a crux of the act, unlike Comprehensive where children have to attend a certain school.
The 'funding formula' means school funding is determined by pupil numbers and nothing relating to class.