Almost 80% of pupils in England who do not achieve a C grade in GCSE maths or English fail to attain this mark during their resits. It is leaving hundreds of thousands of students stuck in a cycle of exams.
"I've failed my maths GCSE four times. It's horrible because you feel like you're stupid.
"You feel like there's something wrong with you. I'm 18, and I'm being put into a class with 15-year-olds," Georgina Tomassi tells the BBC's Victoria Derbyshire programme.
She is desperate to achieve a grade C in maths, after missing out by just a few marks on more than one occasion.
In 2013, the government introduced a policy that said students in England who fail to get a grade C or above in GCSE maths or English should carry on studying the subject, or subjects, until the age of 18, with the aim of achieving this mark.
It means hundreds of thousands of pupils like Georgina - who is also studying for A-levels in drama and health and social care - are taking resits up to twice an academic year.
Figures from the Department for Education show that 77.3% of students in England do not attain a C grade in English or maths when they resit the exam post-16.
"You've got to keep going because I need it to get a job and get into university," she says.
"I'm so close and it's so frustrating."
At Tolworth Girls' School's sixth form, in Kingston upon Thames in south-west London, Georgina and her friends are taught resit classes, but the teachers' busy timetables mean they are limited to just a couple of hours per week.
Chloe Gatt, the school's head of English, says budget strains mean it is difficult to find enough staff to cover those teaching the extra lessons.
Were she not teaching the resit class, she would probably be with her Year 7 or 8 pupils, or her A-level students, she says.
At City College Norwich, just under half of all new students arrive without a C grade in maths or English.
The college's head of school for GCSEs, English and maths, Ray Cameron-Goodman, says it has seen a 440% rise in the number of students taking a GCSE in the past few years.
"In terms of staffing resource, that comes to many hundreds of thousands of pounds every year," he explains.
Then there is the amount spent on entering each pupil for their resit exam - usually more than £30 per paper - and extra, hidden costs.
Because it has so many pupils retaking exams, City College Norwich has to hire Norfolk Showground, one of the largest indoor spaces in the county.
"The cost of the showground alone is about £50,000 - then there's the cost of the transport, the first aiders, the catering," Mr Cameron-Goodman explains.
The Association of Colleges says that in England last year, one in five colleges planned to hire external venues to cope with the numbers.
Two-thirds of colleges were forced to take on extra short-term staff to teach those taking resits, it adds.
Colleges say there is no additional funding from the government to cover such costs.
For some pupils, the resits can feel like an unwelcome distraction.
City College Norwich offers many vocational subjects, such as cooking, photography and hairdressing.
Josh Bennett, 16, is retaking English. When he leaves education, he hopes to work as a bricklayer.
"I'm more of a hands-on sort of person. I've got eight out of nine distinctions in this course so far.
"I find it very difficult sitting behind a desk and doing something like [studying Shakespeare]. I'd rather be outside and laying bricks, laying concrete - and I'm good at it."
Ryan Eves, aged 20, has taken his English GCSE five times without achieving the elusive C grade.
"It's almost a slight bit of torture. They know that some people just don't get English.
"I've tried so hard just to get a letter on a piece of paper."
Ryan has now been offered an unconditional place at university, and no longer needs a C in English - a fact he describes as "annoying", having spent so much time on the subject.
Mr Cameron-Goodman says the government's policy is "a fantastic thing in principle", but is calling for an alternative set of GCSE qualifications to be made available to students who are consistently unable to reach the required C grades.
He says it is wrong to expect every pupil to achieve this mark, as the exam system is not built in this way.
"There is an expectation by the exam boards themselves that a number of students will not pass the examination, and will not pass the examination no matter how many times they resit that examination, so the two things aren't sitting well together."
A Department for Education spokesman said it was "developing credible, high-quality options for students through reforming Functional Skills qualifications in maths and English, to make sure that they deliver the knowledge and skills that employers need, and consequently have credibility and prestige in the jobs market".
Watch the Victoria Derbyshire programme on weekdays between 09:00 and 11:00 on BBC Two and the BBC News Channel.