Prioritize and only put significant effort into things that are interesting to you, require extra depth, or require a complex thought process.
The last two years of high school are actually just enough time to bring up a GPA.
By prioritizing, you can rattle off the busy-work and focus effort and anxiety toward things like large projects, papers, or analyzing. Note that unless you're a science wizard, any science subjects will require extra effort. Not always the case with mathematics.
And something that gets easier in the last years of high school is utilizing empty time, especially if you have empty periods (because there are no classes left for you to take). Try to get all homework done at school, or at least the maximum (the minimum can be done before you class the next day). Keep telling yourself that "homework" is not for "home".
Prioritizing is the easiest system and if you keep turning work in on a regular basis, teachers (at least in my case) will cut you a break towards the end of the year when...um...you just stop doing any work at all. Doing homework all at school is more difficult, because high schools have the tendency to kick you out at the end of the day and unless you have a car, there isn't a place for you to work other than at home.
And you will never ever catch up on sleep. If you can refine the art of sleeping in public spaces, do so, because it's really useful. Polyphasic sleep isn't really an option in high school, but definitely something to consider after those four torturous years.