The statistics showed that a sixth-form’s A-level results were linked directly to its size. The findings call into question Government plans for allowing more schools to open their own sixth-forms, a move which many believe will create more small centres.

Pupils at centres with more than 250 students scored the equivalent of an extra A-level at grade A, compared with pupils in sixth-forms with fewer than 50 students.

The figures are revealed in a Parliamentary answer from Jacqui Smith, schools minister, to Kelvin Hopkins, Labour MP for Luton North. Mr Hopkins said: “This confirms my suspicions that small school sixth-forms can let down parents and pupils and are not good value for money.

“Exam results prove that bigger is better when it comes to sixth forms.

“These figures also raise serious doubts about the Government’s ambitions to increase the number of school sixth forms, made easier by the Education and Inspections Bill.”

Last year pupils in sixth forms with 50 or fewer students scored an average of 169 A-Level points.

But teenagers in sixth-forms with 250 students or more scored 301.6 points on average. This was more than 130 points above their peers in smaller centres. An A-grade at A-Level is worth 120 points.

The Association of Colleges echoed Mr Hopkins’s concerns.

John Brennan, AoC chief executive, said: “Small sixth forms are unable to provide the quality of specialist teaching and choice of subjects larger institutions can. Many colleges offer high-quality teaching in up to 40 different A-levels - small sixth forms just cannot match and often lack expertise in key areas.

“This is not an argument about colleges versus schools - it’s an argument about making sure young people get the very best deal.”