From personal experience (Chemistry), keep it in mind that your school may have specific assessment tasks based on their chosen elective or write their trial in such a way that doing an elective based purely on private study is impossible or simply not worth the effort. Personally, I don't think it's really worth doing another elective based only on independent learning if you have to sit in class and learn the school chosen elective, because it's a bit of a waste of time. Also, your teacher may have valuable resources that will enable you to do better. However, if you can get your teacher to allow you to use lesson time to study your chosen elective, go for it, if you think you are a good independent learner. Keep in mind that sometimes it's up to the head of department, so maybe talk to them too.
Echoing strawberrye, if you want to change elective, you should try and get the whole class option changed, but even then, it's no guarantee.