South Korean students finish Welsh GCSE maths paper in 15 minutes
Three Welsh teenagers swapped school life in Pembrokeshire for lessons in South Korea to find out why the country's students perform so well in Pisa tests.
The tests involve 15-year-olds in more than 70 different countries sitting the same exams in maths, science and reading, every three years.
One obvious difference between the education systems is the long hours South Korean pupils put in.
Sunday Times education editor Sian Griffiths, who took the trio to Seoul, gave pupils there a Welsh GCSE maths exam to sit.
Questions were selected to form an hour-long paper, but some of the local students finished in just 15 minutes.
You can see School Swap: Korea Style between Saturday 25 March and Tuesday 28 March at these times on BBC World News.