As long as they are able to cope with the GCSE level, it isn't intrinsically bad - BUT - to narrow down your options so early could mean missing out on things you would have been great at.
Music is one of those, as the level of improvement in playing/composition and just enjoying yourself in lessons, especially if the only playing you get is using a school guitar, bass, piano or computer at lunchtime - or singing in the after school choir - could be an abject fail in year 9, but a grade 8 by year 11. And most boys have finished their voice breaking by halfway through year 11 - in year 9/10, it could be about to happen, not completed, and you do have kids who sound terrible one Friday, but come back to school on Monday with a bloody good Baritone or Tenor and they're shocked by their change, so need more support/tuition.
We had one go from whiny, nasal voice to a full blown Bass. He would have got next to nothing on his performances beforehand, but after - and because there was time to work with him to develop his new voice and confidence - his was a top rate performance.
Of course, the child of a piano teacher, who has been hothoused and got grade 8 by age 6, will be able to pass GCSE early. It doesn't mean that they should. But they could if they wanted to - if there is a reward, such as not having to take another academic subject, having free study (playing music, drawing, reading, using the sports equipment) or being able to do something that interests them instead.
My opinion might not count in some eyes, though - my GCSEs had a lot of coursework, English, English Lit and another were 100% coursework, so I had 3 under my belt (and two more were good passes if I answered half the exam questions) before I ever walked into the exam hall. So I was quite relaxed at exam time. I would have thought that was a good thing - I got top grades in all my subjects, including the ones that were 100% final exams, too.