Alternatively, you could interpret the numbers as:
Academies: 23.7% to 35%, an increase of 11.3 percentage points
Comps: 20% to 30.7%, an increase of 10.7 percentage points
Independent schools: 44% to 58%, an increase of 14 percentage points.
Or even:
Academies: 23.7% to 35%, so 14.8% of those who previously did not get top grades improved into the top bucket (35% - 23.7%) / (100% - 23.7%)
Comps: 20% to 30.7%, so 13.4% of those who previously did not get top grades improved into the top bucket (30.7% - 20%) / (100% - 20%)
Independent schools: 44% to 58%, so 25% of those who previously did not get top grades improved into the top bucket (58% - 44%) / (100% - 44%)
Lies, damned lies, and statistics, indeed.