Some kids take the initiative or just fall in love with certain subjects before the subject is taught in school. If they can then prove through a test that they already know the subject they should be allowed to skip it to free up time to benefit from other subjects.
Many kids after a few years of learning know which subjects that they like, and they should be able to choose to skip some subjects that they hate. Most subjects that students dislike they don't pay attention in due to them being so boring. That way with more choice they can do better in school.
I think it is a bad idea to allow children to skip subjects as children should be exposed to a wide variety of areas growing up. Children should know about everything that there is to know about so that they can become good at one subject as they get older, but only after seeing a lot of different subjects.
While all of us, when in school, would have loved to have the option to skip certain subjects at school, it's important to get a well-rounded education. Children would otherwise often take what comes naturally to them, or which courses their friends were in. Once children are in high school, they will be given some opportunity to pick and choose which courses they wish to take. College will also afford this opportunity. Until then, it's important for all children to learn the basics about subjects like science, math, and literature.