It's quite common for pupils to get one or two GCSEs out of the way in year 10. My son has just done two. Even 30 years ago I did my English Language in year 10
Going back to my own teen years, I remember a girl who got an O'level in Serbo-Croat (don't ask) in the equivalent of year 9. She took the exam externally, via her Yugoslavian community association.