I don't think any of the subjects are generally unnecessary for students. Not everyone is going to end up going the same path and schools offer the education that fits everyone's levels as well as providing different extra curricular activities to help decide what you want to continue doing in the future.
I totally agree with Drox's post above. Also, if you really think you're gonna struggle starting out as an adult because school doesn't teach you the basics like billing, credit cards, whatever, then I suggest taking a gap year. Within that year, you'd have the time to prepare yourself for the real world.
