I really don't think art should be compulsory.
The thing is with art and art related jobs is that you don't actually really need a degree in art to be successful- you could be completely unqualified. However the reason why some people want straight A*s or really good GCSE grades is because they want to have jobs such as being a banker, doctor or lawyer etc and to be successful at that you generally speaking ideally need to get very good exam results and go to a very good university. There are exceptions, but this is the case for most.
So what would be the point of a compulsory art GCSE? Not only do some of us find art terrible (I for one always hated it with a passion), but it is also a lot more about skill than hard work. If someone is naturally talented at art they could achieve an easy A* whereas is someone was naturally good at a subject such as Biology they still wouldn't do very well unless they put the time in to revise and learn all the exam content. Art is also not the most useful of GCSEs because as I said, to be a successful artist you don't need an A* in art for GCSE and students who can work really hard to get straight A*s can't necessarily be capable of gettin an A* in art so all their hard work would still lead to a bad exam grade in one subject.
At the end of the day whilst art would be enjoyable for those who like it at GCSE, I don't think it really is particularly useful in day to day life and theres no point making people who hate it study it. Subjects like History or modern foreign languages arent compulsory in some schools and theyre far more useful so I believe that art should never be compulsory to students, although it is good to have as an option to those who want to study it.