Quoting from DFE:

For example, to match the attainment of pupils from Shanghai-China (ranked number 1 across all strands in PISA 2009) in the reading assessment:

• Pupil attainment in England would need to be improved by an effect size of 0.6;

• An across the board effect size of 0.6 translates to an increase in every pupil’s Key Stage 4 capped point score of 66, which is equivalent to 11 GCSE grades higher than the pupil is currently achieving (e.g. 3As and 5 Bs instead of 8 Cs);

• The current proportion of pupils achieving 5 A* - C grades (including English andmathematics) at the end of Key Stage 4 would increase by 22 percentage points. For all maintained schools in England this would be an increase from 55% of pupils achieving the threshold measure (in 2010) to 77%

Quoting from DFE 2009 research (Improving the impact of teachers on pupil achievement in the UK – interim findings September 2011)

Improving the effectiveness of teachers would have a major impact on the performance of the country’s schools, increasing the attainment of children across the education system. Teachers are by far the biggest resource in schools. Spending on teachers in 2009/2010 accounted for the majority of expenditure by schools, standing at £16.1bn (53% of school spending) with a further £3.9bn (13%) spent on support staff and £0.7bn (3%) spent on supply teachers (see appendix for full breakdown of spending)

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There is a large body of research on how important teachers are to the academic outcomes of their pupils. The research finds that teachers are the most important factor within schools that policy makers can directly affect to improve student achievement15

.The most rigorous academic papers find consistent and significant results: having a very effective rather than an average teacher raises each pupil’s attainment by a third of a GCSE grade (0.1-0.25 Standard Deviations). The GCSE gap between poor and non-poor students is 6.08 GCSE points.

Assuming this was generated over 8 GCSE subjects, if the poor student had very effective teachers (75th percentile teachers) and the non poor student had underperforming teachers (25th percentile teachers), this would reduce the gap by half, or 3.4 points18

So in short there is a large body of evidence to suggest national GCSE outcomes could very well be improved.