Inspectors failed the school last November because of poor teaching, low expectations and pupils' lack of progress in English, maths and science.
James Allen's prep school has 18 pupils per class. Local middle-class parents struggle to get their children into pound;5,000 a year because of its high standards.
But next term, pupils will learn maths together as part of the Government's programme.
In the first half of the term, a specialist teacher, funded by the scheme will give three demonstration "numeracy hour" lessons to five to seven-year-olds in each school each week. In the second half, 12 six-year-olds from each school will be taught as a class for half a day a week, first in one school, then in the other.