the reality is, if you do it, your GPA probably will fall because you'll have too much free time on your hands and staying focused on doing well in school will become harder.
I understand that being...what 15 or 16 right now that probably sounds fucking awesome to have tons of free time and yada yada, but your time will be better spent in school, thus increasing your chances of doing well. I was able to get 2 full classes worth of college credit knocked out my senior year with AP stuff, and it made my life way easier when I needed it in college a couple years later.
GPA isn't a golden ticket to college, but it's pretty critical if you want to CHOOSE where you go to college vs. going wherever will take you. And furthermore, if you get in the habit or working hard now and doing well academically, you'll most likely maintain that habit in college - which will benefit you for the rest of your life.
I fully realize that I sound old as fuck writing this stuff, and at 26 years old, I probably am compared to most of you guys. I'll tell you what though, what I'm saying is the truth. At the end of the day it's your choice of course, but really think it through, try and see the big picture instead of the little picture where 3 extra hours of free time a day seems like the best thing ever. Just my .02
tl;dr ... hope it helps somebody