I also think that you should take the class that is right for you. It sounds like this is chorus. Taking the classes that are right for you is going to make it more likely that you will get accepted to universities that are right for you (and yes this approach has worked well for us).
MIT is STEM heavy.
Different high schools in the US calculate GPA in very different ways. Top schools such as MIT and top ranked liberal arts colleges will look at your grades. I would not worry about how your high school computes GPA.