Hats off to you! I know people in their 40s who STILL havent figured out (or made any effort to figure out) how to budget their time properly. Its never too early to start learning that - they required everyone in college take a class first semester on time management, etc. And I sat through it thinking "Good lord I've been doing this stuff since high school - and why the hell didn't we learn it in GRADE school instead?!"
Sit down and write out a schedule for your day. Alot adequate time for homework and then put in riding time where it fits. Leave yourself some spaces of catch up time - either for riding or homework - or doing whatever you need to relax. There are lots of computer programs or schedule books out there.
Extra un-used homework time can be used to get ahead on a project to create more riding time! Keep a project list up somewhere where you can see it and list projects and due dates on a time table. Make an "in" and "out box set on your desk for paper assignments; you'll ease stress by being able to look at the "out" box and see just what you've accomplished.
I have never met a teacher who had a problem with a project being handed in early! My mom and stapfather are both retired school teachers and thats one complaint about students they NEVER had! If its really an issue for a teacher, well, it can sit in that "out" box until due day, and get checked off your list.
I always alotted myself a study hall period every semester - if you know the goal is to use that time so you can be free after school, its amazing how much you can get done with 45 minutes of you time in the middle of the school day.
I also made use of ANY free time I had in class. If there was time at the end of class to do homework, I did mine. If I finished my work and still had 5 minutes, well then I pulled out an assignment from a different class and buckled down. Or I read or wrote to a friend. It might sound weird but since I never got in the habit of gabbing with friends during class time, I never offended them or felt like I was "missing out" by using that time to do my work. I had friends, we talked before and after school, between classes, we wrote notes to eachother. But sometimes to be a horse kid and a good student, you have to know when to stay focused.