I don't think any of its useless really, I mean when else in life do you get to learn new things for free for eight hours a day?
Sure some is mundane and force-fed, often presented by pricks and low-lifes, but as I see it, when I'm out of college and in the work force full-time, I will be sad I don't have such good opprotunities to learn something, anything really. Everything you learn helps no matter what, in one way or another.
Now one subject I have a problem with is history. It never really struck me as an honest course, as it always makes the US seem like the best country in the world. I mean obviously, since its being taught in a school funded by the government, so it would be intended to make you appreciate and respect the government, but text books are just plain screwed up. For example, remember how in 2nd grade you learned how awesome Christopher Colombus was? And how every year after that he turned into more and mroe of a douche bag, bringing disease, enslaving natives, raping and pillaging et cetera et cetera. I can't imagine how much fo our own government's secret dealings are hidden from us. It's important to understand the past, but I don't think we achieve any level of understanding with high school history.