maybe if you have volunteering say 4 times a week you can cut it down to 2 times a week its your last year of all that high school stuff dont worry it will be over soon then you will have the freedom to make your own time hang in there :)
Sleep, is your most important asset when overworked (else, your efficiency may plummet and you will fail one or more of your courses).
Cut back on TV/PC/videogames to ensure you're getting ample sleep. At your age, at least eight hours of good quality sleep.
You may have taken on too much, and have to drop one or two discretionary school activities.
We all have 24 hours, from billionaires to street beggars, no one has one second longer.
It's how we use our time that makes us different. Some things like homework, can be done in bits and pieces (so, carry a task with you so that when you're at loose ends to fill your time, you can plug away at some homework task). Make more minutes of your day count for something rather than frittering them away.
I'm currently volunteering 1x a week (2 hours) and dancing 3x a week (1 - 1 1/2 hours). I'm at least getting 8 hours a sleep each night, 9 hours if I'm lucky and don't have too much time for any TV/Video Games.
I've already quit soccer for a year since I didn't have any time and am wanting to do my school's spring play (if there will be one). I'm not really good at managing time.
You're almost there, almost done. Just make it work now and in a few months you're going to graduate. I know you're having a hard time but it's all about sacrifice. Create a schedule so you have something to follow it would really be beneficial if you can commit to your schedule. You'll have plenty of time to utilize. Don't burnout yourself, take breaks and unwind sometimes but also don't enjoy too much. You can do that once you graduate.