I was raised with horses but got my own Keeper horse when I was 12. She was in foal but nobody knew it. I kept that foal until I had to lay him to rest when he was 29 and I was 42.
Since I am old enough to be your grandma, I will repeat what my mom said to me when we bought Goldie:
"This is your responsibility - every day - unless school activities keep you from getting home at a reasonable time to feed the horse - unless you're too sick to get out of bed and go to school, THEN AND ONLY THEN will I take care of the horse for you. IF you don't take care of the horse, you will come home from school one day and find the pasture empty and the next time you get a horse, it will be when you have a job and can pay for everything yourself"
Even though I'm an Old Timer, I can still pretty much remember those words verbatim - lol
I think I missed one day of feeding from the time I was 12 until I moved out at 18, and that's because I really was too sick to get out of bed.
Mom was an animal lover and far from being mean, she just wanted to be sure I fully understood the full scope of responsibility of caring for a horse and learning how to manage my time wisely. If something happened and we needed the vet, I never had to beg, she was on the phone while I was still at the barn.
As far as sports and academics? I played softball, was a basketball cheerleader (my very rural school was too small for football) and I graduated high school with a 3.89 cumulative.
If you want the horse bad enough (meaning there's a degree of horse addiction coursing thru your veins:), stick to your end of the bargain by being diligent about caring for the horse and keeping your grades up. If your parents think you should work to help earn money for its care and keeping, get busy
I spent summers putting up hay on my granddad's farm. He raised beefers and Welsh/Morgan crosses so there was a lot of hay coming off those fields for a young lady to work off feed for the winter
I loved my horse (eventually two when Sonny was born) enough to devote all my spare time to their well-being, which wasn't much because my parents skillfully always had just enough things for me to do that I was busy until bedtime - lol lol
The surprise foal that nobody knew the mare was carrying stayed with me 29 years, as I've already stated. That means I found a way to keep him, get a job with General Motors and work a boatload of overtime, get married, have a baby, get divorced, be a single mom and make sure the child got his homework done, played a bit of sports in school, and STILL take premier care of that foal who taught many children to ride in his later years.
YOU have to decide where your drive and ambition is. If the horse is just a passing fancy, wait until you're on your own and can afford one; if it's a born-in-the-blood addiction (like it was with me) you will find a way to manage your time and make it work.
You asked, so there's first-hand experience from an Old Lady who's only been out of horses one year of my life since I was 12
That was the year I laid the surprise foal to rest, when he was 29, and I was so devastated I didn't want to think about owning another horse.