Schools which cheat the system by entering children for easy GCSEs face a fresh crackdown, the new Ofsted chief will announce on Friday.
Amanda Spielman will use her first key note speech as chief inspector of schools in England to warn that the situation in some schools is "nothing short of a scandal".
The head of the watchdog will use her address to the Association of School and College Leaders' annual conference to announce a major Ofsted investigation into how schools are ensuring that children receive a broad education.
Pressure to succeed in league tables is leading some schools to narrow the national curriculum and "move out" pupils who threaten to bring down results, she will claim.
The announcement follows a warning from Ofsted earlier this month that an increasing number of schools are entering pupils for non-academic qualifications, which aren’t in the best interests of pupils, in order to boost their performance data.
Ms Spielman is expected to tell the conference in Birmingham: "We know that there are some schools that are narrowing the curriculum, using qualifications inappropriately, and moving out pupils who would drag down results.
"That is nothing short of a scandal. Childhood isn't deferrable; young people get one opportunity to learn in school; and we owe it to them to make sure they all get an education that is broad, rich and deep.
"There is more to a good education than league tables. Vitally important though a school's examination results are, we must not allow curricula to be driven just by SATs, GCSEs and A-Levels."
We know that there are some schools that are narrowing the curriculum, using qualifications inappropriately, and moving out pupils who would drag down results. That is nothing short of a scandal
The Ofsted head will add: "It is the substance of education that ultimately creates and changes life chances, not grade stickers from exams.
"So I am determined to make sure that the curriculum receives the proper focus it deserves."
Ms Spielman is expected to say pressures of accountability are leading to a conflict between headteachers' desire to give pupils the right education and the desire to improve league table positions.
She will also make clear that she does not believe the curriculum has received enough attention during inspections in recent years. Inspectors will carry out fieldwork for the investigation over the next few months, before the final report is published later in the year.