I graduated in '08.. got my first horse about 3 weeks after I started 9th grade. It was definitely nice having him to go ride, groom and take care of when I was having a bad day.
But, I will say, my grades would have been better without him ;)
Not that I got bad grades - mainly As and Bs, maybe a C in math a few semesters. But if I only had to worry about school work then I would have had straight As, I'm sure of it. I'm just the type of person who thinks my horses are the most important thing ever and everything else takes a back seat to them.
I'm doing the same thing in my college courses... I have a 3.5 GPA overall and a 3.0 this semester, but without horses sitting in my backyard it could've and would've been closer to 4.0 :P
You just have to stay focused and know how to manage your time effectively. Ride when you can and on days you have a lot of school work/projects/reports maybe just go groom :)