Michael Gove: "The situation we have at the moment is flawed"
Tory plans could see schools get extra league table points by moving more pupils through what they call "harder" A-levels, such as maths and physics.
The proposals for England also suggest awarding fewer points for subjects seen as easier, such as media studies.
The Conservatives claim the exam system has been "dumbed down" and that league tables are partly to blame.
Ministers are changing the league table system but say subjects are "rigorously measured against each other" already.
Michael Gove, the shadow education secretary, told the BBC's Andrew Marr Show that he felt the current system of ranking schools on the proportion of students attaining a C grade and above at GCSE was flawed.
He said teachers felt pressured to concentrate on borderline C-grade pupils while the needs of more able students were ignored.
"The truly brighter students aren't being stretched because there's no emphasis on getting people from a B to an A or an A to an A*.
"And what's even worse in my view is that those weaker students, who could really benefit from extra care and attention aren't focused on either."
His party argues that pressure for schools to perform in the league tables could mean pupils are pushed to take subjects that might be easier in order to achieve a higher rank.
The Tory proposals, the first to emerge from an inquiry into the examination system led by Sir Richard Sykes, the former rector of Imperial College, London, also include removing vocational qualifications from league-table rankings.
The Conservatives claim some exams are easier to pass than others
This would include the government's flagship new diploma, which Conservatives claim is "nowhere near as academically demanding" as traditional A-levels and GCSEs.
Schools would also no longer be judged on the proportion of pupils who gain five A* to C grades.
Mr Gove said: "We believe that it's right that there should be something closer to a points system so that there are set number of points for an A*, fewer for an A and so on.
So that the effort of all is rewarded and schools genuinely get credited and recognised for doing well."
There is objective evidence from people who care about academic standards that they are not what they should be
Michael Gove Shadow education secretary
Instead, a points-based system would be introduced, placing more value on higher grades.
Mr Gove also said universities no longer considered every A-level to be equally rigorous.
Critics say practical qualifications, such as cake decoration, pottery and flower arranging are being given equivalent value to traditional A-levels.
One example given included a course in "tanning treatments" which was worth 45 points in school league table scores - the same as an A grade in one of the four units that make up an A-level.
At the same time, Mr Gove said, exams which are rigorous, such as the international GCSE (IGCSE), do not count at all in league tables.
Mr Gove warned that certain universities such as Cambridge, the London School of Economics and others had told prospective students that taking "softer" A-levels such as media studies and dance would count against them.
He also said independent reports from the Royal Society of Chemistry and Sir Peter Williams, who carried out the government's maths review, showed there had been an overall fall in standards.
"There is objective evidence from people who care about academic standards that they are not what they should be."
BBC political correspondent Laura Kuenssberg said that with A-level results due out in a few days, the subject of standards was of "great interest to families up and down the country".
But, she said, the government was already planning to replace league tables with an overall grade for each school that would consider things such as children's behaviour as well as exam results.
The government has dismissed the Tory proposals.
A spokeswoman for the Department of Schools, Families and Children said: "We simply don't recognise the labels 'soft' or 'hard' A-levels - all subjects are rigorously measured against each other to maintain standards, overseen by Ofqual."
More young people were staying on at school to take A-levels, which was "something to celebrate", she added.
"Take-up in science and maths A-levels is continuing to rise and schools are also offering a much broader range of qualifications - including the Diploma."
League tables have already been scrapped in Scotland, Northern Ireland and Wales.