The success of the private sector is down to its independence, say researchers
Independent schools get better results than state schools because they have the freedom to tailor teaching to the needs of their pupils, researchers say.
A University of Buckingham report found social background and ability were not the only factors behind higher grades in private schools.
The study said autonomy meant decisions were made close to the classroom.
The findings showed how the quality of education could be improved in the state school sector, the report added.
The study was commissioned by the Headmasters' and Headmistresses' Conference (HMC), which represents 250 independent schools across the UK.
It found the academic record of HMC schools outstripped that of state schools, including grammar schools, but suggests this cannot be attributed to social advantage or to their selection process.
Grammar schools were more selective, the report argued and yet HMC schools outperformed them in exams.
It found that HMC schools achieve better A-level results in 24 out of 30 subjects than grammar schools and in 29 out of 30 subjects than comprehensive schools and academies.
Freedom to innovate
Report author Professor Alan Smithers said the autonomy enjoyed by the independent sector meant decisions were made "closer to the teaching".
"The autonomy of the school is very effective in delivering high quality education," said Prof Smithers.
"What we really need to do is make the shape of the system right, put in the money, then have the courage to step back and let teachers take the decisions.
"It gives an indication to the government as to how they can improve the quality of education in the state sector."
Schools Minister Andrew Adonis said freedom for schools to manage their affairs was a "key principle" of government policy.
"That is why we have established 130 academies - which are independent state schools - and given all schools greater control of their budgets, staffing and management."
The research also found that independent schools were likely to recruit and retain the best teachers.
Three times as many teachers moved from the state sector to work in independent schools, attracted by the greater freedoms to teach how and what they wanted.
"We found that teachers in independent schools were more likely to have a degree in the subject they were teaching and to hold a good degree from a leading university," the report said.
"Not being bound by national pay scales gives them [independent schools] the freedom to do what is necessary to recruit teachers of the quality required from whatever sources are available.
"Bearing in mind the importance of teacher expertise to student outcomes, it is reasonable to infer that a key factor in the quality of education provided by independent schools is the teachers they are able to attract."
The report likened private schools to businesses, with parents being customers.
"It is not a matter of running up a deficit and having knuckles rapped by the government or local authority.
"If they do not turn a profit then that's it."