That's rough, I know how it is to have to juggle school (and later college) work with my horsey preferences and obligations, but I was lucky in high school to be homeschooled. It is important to remember that an education is really important, and not to skimp on that, but there are a few tips I can give you on maximizing your time.
I did lots of homework and studying on the bus rides to and from school. I had about a 45 minute jumpstart before I got home. I would do the rest immediately after I got home, when I was still in a "work" frame of mind, then I had the rest of the evening to be in the barn, watch TV (in my pre-horse days...), read, whatever. Year planners help, too. It's easier to keep track of what is due when and to decide what takes priority within your homework. Get what you have least time until it's due or what is the most time consuming done first.
Also, I can't help but recommend homeschooling to you. I know it isn't for everyone and every situation, but there is a really surprising amount of time wasted in public school. After I started homeschooling I was literally done with 6 subjects work in three hours a day, and almost every other homeschooler I know had a similar experience. It won't hurt your college hunt process, either. I managed to get into a really competitive private college on my homeschooled high school education. Feel free to PM me if you have questions about homeschooling at all.
Anyway, education is really important, but there are definitely ways to maximize and organize your time and still get to the barn. 2 hours of homework doesn't floor me nearly so much as 12 tests in a week. Wow. Do you have that many subjects? How did that happen??
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown