A leading headmaster has condemned the "corrosive" influence of league tables and a lack of careers advice on schools.
Dr Tim Hands, master of private Magdalen College School, Oxford, said politicians pay too much attention to statistics rather than individual students.
Speaking at a Westminster Education Forum on higher education in central London, Dr Hands said he was "horrified" when he was told by a pupil that at their previous school they were told not to work so hard as they only needed to achieve a C grade.
Under the current system, state schools are judged on the number of pupils who score at least five C grades at GCSE, including English and maths.
This measure is included in league tables, with schools expected to have at least 40% of students reaching this standard. Those which do not, and fall short on other pupil progress measures, are considered failing.
At the education forum it was suggested by a delegate that many students from working class backgrounds have high aspirations to study subjects such as medicine, but do not have the A-levels needed for these courses. Many state schools are not aware that certain A-levels are needed for some degree courses at an early enough stage, it was suggested.
Dr Hands said: "There are twin corrosive influences. One is the withdrawal of careers advice, the second is the emphasis on league tables and C grades being the only important indicator.
"Personally, I interview every pupil in my school in Year 11 about what career they might think about doing and what are the right subjects for that. I also teach everyone in Year 12 who does A-level English. We have an extensive bursary system into our sixth form for people who want to come into that area of expertise.
"I was horrified when a girl who I was teaching who I asked about her experiences at previous schools said the difference here is that 'I'm being asked to achieve my potential. At my previous school I was told not to work so hard - you don't need an A, you only need a C'.
"Why is that the case? Because the emphasis from politicians is on league tables and on statistics rather than on individual good. It's perfectly simple."
