The gap in A-level performance between schools and sixth-form colleges has halved in the past two years, official figures show.
School students now gain an average of only one half-grade higher than their peers at sixth- form colleges.
All colleges outperform school sixth forms with fewer than 150 students.
Evidence of colleges' improved performance follows the Government's announcement that it will allow an expansion of school sixth forms.
The proposals, contained in its five-year plan, will speed up applications from high-performing specialist schools keen to set up their own post-16 provision.
Sixth-form colleges fear they will be left at a disadvantage as more schools compete for students, despite ministers insisting that sixth-form colleges are an important part of the post-16 mix.
The Sixth-Form College Federation has said schools have an unfair advantage because they can spread administrative costs across the school and can persuade pupils to stay on at 16.
The figures comparing A-level performance were revealed in Parliamentary answers by David Miliband, school standards minister. They show school pupils gain, on average, the equivalent of three grade Cs at A-level and a B at AS-level. This compares to three Cs at A-level and a C at AS-level for students at sixth-form colleges.
But there are wide variations between school sixth forms of different sizes.
Those with more than 250 pupils do best - pupils gain two grades at A-level higher than the average for all schools (two grade Bs and a C at A-level and a B at AS-level). By contrast, students in sixth forms with fewer than 50 pupils gain only three grade Cs at A-level, although their results have improved even faster than those of sixth-form colleges in the past two years.
Students in further education colleges lag further behind, with only three grade Ds at A-level.
John Brennan, chief executive of the Association of Colleges, said money earmarked for opening new school sixth forms should be used to improve funding of sixth-form colleges.
He said: "Small sixth forms are unable to provide the quality of specialist teaching and choice of subjects which larger institutions can, and these figures show that only too clearly.
"The Government should be very cautious in its plans to create new small-school sixth forms, as they will not necessarily benefit the students that it wants to help. These figures show once again that small-school sixth forms can give them a raw deal."
John Dunford, Secondary Heads Association general secretary, said:
"Improvements in the results of small sixth forms show the debate has moved beyond what is a viable size for them.
"What is important is not size but quality."
AoC conference, FE focus 5