The students skip the AS-level exams, taken after the first year of study, and sit all papers at the end of the second year.
Students at the pound;5,470-a-term private school in Abingdon, Oxfordshire, consequently face up to 24 exams this month.
While coursework relieves some of the pressure, the 125 students have to cope with a maximum of six modular exams, between one and three hours long, in each of four subjects. The pay-off for such a demanding June is more time spent in the classroom and an exam-free lower sixth.
Headteacher Angus McPhail said: "We estimated that if we put students in for AS-levels, six weeks of teaching time would have been lost with a combination of revision, mock exams and resits."
One drawback is that students do not get a second bite of the cherry. They cannot resit an AS exam, if they performed badly, within school time.
But the school's results - 100 per cent pass rate and more than three-quarters achieving A and B grades at A-level - mean resits are not generally needed.