The exams system is "a statistical monster that no-one seems to understand", a teachers' union has said.
And new qualifications - such as the English Baccalaureate - will not do enough to solve "deep problems" that exist in the assessment of pupils, according to the National Association of Head Teachers (NAHT).
Russell Hobby, the NAHT general secretary, said the union's 28,500 members in England, Wales and Northern Ireland are demanding politicians tackle flaws in the education system as they approach the party conference season.
Unveiling the NAHT's aims for the 2012-13 school year, Mr Hobby said the recent GCSE grades crisis is just one example of a wider problem.
"We have seen the recent disruption caused to many young people's lives by the decision to change GCSE grade boundaries mid-way through the academic year," he said.
"This alone has thrown up wider concerns about the state of school assessment across all phases of education from five to 18. The exam system has become a statistical monster that no one seems to understand, be able to predict or control."
He said teachers were under pressure to "teach to the test", and an "exam optimisation" has developed in schools, which narrow and impoverish education.
Mr Hobby said "perverse incentives" for teachers lead to "wrong choices" being made for students, such as early entry to exams.
The general secretary welcomed the introduction of the English Baccalaureate, but said it did not tackle the problem that already exists in schools, that pupils are not assessed on participation in a wider spectrum of disciplines, such as sport and music. He said: "The award of a complete certificate for the English Baccalaureate should require evidence of broader activity in sport, culture and the arts and civic responsibilities such as volunteering."
A spokesman for the Department for Education said: "We must ensure that our exam system is robust, rigorous and relevant to match the best education systems in the world. We make absolutely no apologies for doing this. Our exam reforms will lead to higher aspirations and greater achievement for all pupils, including the most vulnerable. We have chosen core academic subjects to be part of the English Baccalaureate Certificate and running competitions for these subjects will set the benchmark for the whole system."