Pupils are making more progress at Thomas Alleyne's than at all but one other school in East Staffordshire.
Recently-published school league tables show the town's only high school rated the second-best for progress in the Burton constituency.
Its score of 0.17 was significantly higher than the national average of -0.03.
Head teacher Julie Rudge said: "We are delighted that the school performance league tables show the continued high performance of our students and staff.
"The release shows that, for the third year in a row, students have performed significantly above national and county averages for progress at GCSE.
"Our focus at this point in time is with our current year 11 students who have just completed their mock examinations.
"They have already shown how committed they are to their studies and we are hoping for even better results this summer.
"As a school we maintain our commitment to offering all our students an education which is broad-based, enriching and stimulating."
Alleyne's' "progress eight" score was only bettered by Abbot Beyne School, in Burton, which had less than half the number of GCSE pupils at Alleyne's.
Sixty per cent of entrants got five Cs or above, against a national average of 56 per cent.
The Dove Bank school's "attainment eight" score, which measures performance across eight key subjects, fell slightly below the average across the country.
Despite that, the score was still the fifth-best in the borough.
Pupils' average grade was a C, in line with UK-wide school performances. That feat was matched by JCB Academy.
Both schools matched the national average of 42 per cent for entrants who achieved at least "grade five" - equivalent to a C - in both England and maths.
But the Rocester school's "progress eight" score of -0.84 was judged "well below average".
The specialist engineering school was also below the national average in its attainment eight score.
JCB Academy did not provide a comment on the results.