Teenagers can no longer rely on being talented in one subject to win a place at a top university, a leading headmaster has warned.
The modern GCSE system is a test of students' "character, determination and stamina" because it forces them to work on their weaknesses, according to Dr Tim Hands, master of Magdalen College School, Oxford.
He also called for an end to "political meddling" in the exams system, arguing that it has "rarely been seen to be helpful".
Dr Hands was speaking as Magdalen College, a private boys school, topped a GCSE table of fee-paying schools.
In total, 99.52% of the school's GCSE entries scored an A* or A this summer, the table shows, with pupils notching up 656 A* grades alone.
As the results were published, Dr Hands said it was "absurd to rubbish all the hard work put in by young people to GCSE, deploring the supposed drop in standards, and lamenting the old O-Level system."
He said: "The fact is that in the old days, you could do well in the subjects that you were naturally good at, and secure a place at a top university. That's no longer the case. You won't get into a top university unless you have good grades across the board.
"This means working at your weaknesses and it makes the modern system a test of character, determination and stamina. The truth is that GCSEs are a better and broader set of tests, and more meaningful to employers and universities."
This is a "massive change" that has not been realised, Dr Hands warned, adding that employers and universities are more likely to be interested in candidates who know their weaknesses and are determined to work at them, than those who only work on the areas they are good at.
The headmaster also disagreed with previous suggestions from some education experts that GCSEs should be scrapped and replaced with new tests for 14-year-olds. GCSEs are "a meaningful and important hurdle", Dr Hands said. He also argued that coursework is still a valid method of assessing students.