And she also told BBC Radio Four's Today programme that grant maintained schools remained a very important priority.
Mrs Shephard was responding to criticism of government education policy by two party representatives attending conference. Lecturer and former school teacher Graham Pycock, of Dulwich, said he regretted the focus on grant maintained schools because most schools were locally managed and had increased standards.
Mr Pycock, vice chairman of the Conservative Education Association, added: 'I regret this obsession with grant maintained schools, which are only 5% of the total number of schools. I think grant maintained schools are a way of bringing in selection and the last thing this country needs is a return, a turning of the clock back, to effectively a mixture of effectively grammar schools and local authority secondary moderns'.
But Mrs Shephard said GM schools were an important plank of government policy in increasing choice and diversity for parents. They provided a benchmark.
'It is the case that those involved with grant maintained schools find that the independence from local authorities and the independence in handling their own budgets helps increase standards'.
She added: 'The evidence from our Chief Inspector is that there is no link between attainment and class size. The important thing is the quality of teaching and the quality of leadership'.