More students than ever across the south are achieving high GCSEs, but national figures also show large swings in English and maths results.
Head teachers said some schools were seeing "volatility" in results, warning that for some students, this could put their chances of a place at a top university such as Oxford or Cambridge, or their opportunity to go on to sixth-form college, at risk.
Results for England, Wales and Northern Ireland showed a sharp drop in English grades, with 61.7% of entries scoring A*-C, down 1.9 percentage points from last summer.
Exam chiefs suggested that the changes in results were down to recent education reforms, including removing speaking and listening from final English grades, a decision that in England, making only a teenager's first attempt at an exam count.
In recent years there had been a growing trend towards schools entering pupils for exams early, or multiple times, but the new rule has changed this, and figures published earlier this year showed around a 40% drop in early entry across all subjects.
"What I think is really of note is the change in the 15-year-old results overall," Mr Hall said. "What is driving that is the 'first result counts'. Only the students who are really strong in the school's judgment are being entered at 15, whereas before they were being entered to see how they get on."