Statistics from the Department for Education (DfE) show that 365 mainstream secondary schools (12%) fell below the floor standard past year, up from 282 (9.3%) in 2016.
Official figures show one in eight mainstream secondary schools did not reach the expected scores in 2017.
Kate Pereira, headteacher at St Benedict's, said they were "delighted" with their score.
There were three schools in the borough that were deemed to be underperforming, falling below the minimum level required of -0.5. "It's a freaky random number".
"We are confident that all of our schools are working hard year on year to secure the best possible outcomes for their pupils". The floor target becomes skewed with such a small number when some of the students are not following the normal curriculum. "That reduces the floor target".
Following the publication, the Arthur Terry school reported excellent results for both progress and attainment.
The schools are judged by two new measures, Progress 8 and Attainment 8, based on raw GCSE results from a year ago.
All of the other state secondary schools in Banburyshire are considered "average", while Space Studio Banbury was the only one below the national standard.
Significant weight is given to English and maths.
Shireland Collegiate Academy in Smethwick maintained its position at the top of the pile in the borough, with a performance that moved the school into the top 12 per cent in the country for the progress 8 measure.
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The DfE said the rise in under-performing schools - from 282 schools the year before - is because of technical changes to the points system used by the government to calculate a school's performance.
"Progress from year 10 to 11 - the time students are at the UTC - is exceptionally good".
Another new measure is "Attainment 8" - looking at the raw attainment from pupils in their best eight subjects, including English and maths.
A negative score does not mean pupils made no progress or the school has failed. We have also closed gaps between the most disadvantaged students in school. Dr Mary Bousted, the joint general secretary of the National Education Union, said: "The DfE presents today's results as a sign of continued progress".
It also saw 57 per cent of 164 pupils achieving grade 9-5 in both English and maths at GCSEs.
Many Banburyshire secondaries scored above the English state school average of 46.3, with Chipping Norton School getting the highest of 52.4.
They measure a child's progress and their grades across eight subjects, from the end of primary school to the end of secondary school.
Headteacher Kate Parker-Randall said: "It is a superb set of results and we are really proud of the efforts of our talented and hard-working students".
Keswick School was "average" in Progress 8 with 0.12, Attainment Score 8 was 53.8, Grade Five or Above in English and maths GCSEs was 58% and Achieving EBacc above grade 5/C or above was 32%.