GCSE reforms that make it harder to gain the very top grades could end up “narrowing” pupils’ learning as they focus ever more tightly on exams, a leading independent school headmistress has said.

Alice Phillips, president of the Girls’ Schools Association, said she was concerned that plans for GCSEs to be graded on a scale of 1 to 9 could harm learning as pupils sought exam “perfection”.

Over time, she said, level 9 at GCSE would be what leading universities would expect when making their offers, so that would be what pupils aimed for.

Ms Phillips, who is due to address the annual conference of the GSA today, spoke after the exams regulator Ofqual revealed earlier this term that just 20 per cent of A or A* candidates would be given a grade 9 when new GCSEs in English and maths were first awarded in 2017.

Ms Phillips, who is headmistress of St Catherine’s near Guildford, Surrey, said: “This is going to be your first set of public exams and...you are bound to feel a tension around that.

“If you use the full range of the marks, then perfection is possible. The problem with perfection is, if it’s out there and it becomes aspirational, you will push yourself to get it.

“Aspiration is a great thing but if your whole future is going to hang on 3, 4, 5 per cent of the marks aged 16, I wonder if that isn’t distracting from the real purpose of education, which is to stimulate, inform, broaden and draw out from the young person the best that they can be.”

She said it could have the effect of reducing students’ interest in “that really fascinating additional science experiment” or interesting diversion in an English lesson, because pupils would just be interested in “what they need to know”.

“Teachers are going to be challenged in a different way to try and keep the excitement of the breadth they could bring in, and I think that’s a great shame at age 15 or 16,” she added.

Ms Phillips spoke to TES in advance of her speech to her organisation’s annual conference this afternoon, where she is expected to launch a broadside against GCSE and A-level reforms.

Plans to axe the AS-level as a stepping stone to A-level, she will say, will have the effect of reducing the breadth of study and turning GCSE into an “unforgiving…last chance saloon” that will decide people’s futures at a very young age.

She is also expected to call for education to be distanced from quangos and party politics, helping to free it from “costly changes of direction” and “quick fixes”.

“Children and their future - our country’s future - should not be at the mercy of those for whom education is a means to secure votes and power,” she is due to say.

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GCSE reform: What it will mean for your school: 3 April 2014

Cambridge University urges schools: do not ditch AS-levels : 5 November 2014