Nada Farhoud, who is studying for her A-levels at Wooton Upper School in Bedford, wrote to the Prime Minister after learning she was the only one in her year to apply for an Oxford place.
Six other Oxbridge candidates in her year group have opted to apply to arch-rival Cambridge, claiming Oxford is biased against comprehensive-school applicants.
In her letter, published in today's TES, she criticises Oxford's "daunting" interview process and its system of obliging candidates to apply to individual colleges. She also claims that, after the A-level results are published, Oxford tutors regularly phone round favoured schools to fill vacant places.
Mr Blair, himself an Oxford graduate, has responded to her with a personal letter.
He said: "You are right to raise this issue. Enabling every young person to fulfil their potential is vital for all our futures."
Oxford is considering ways of increasing applications from state schools, while the Government has announced a new scheme to encourage gifted pupils through summer schools at universities.