Sounds like you're a lot more grounded than most of the girls in your school. At the same time, I am sure there are many other girls like you, at the school. I assume that you clean pens at a stable. Have you looked to see if there are any other people your age there who also take care of animals or ride? That might be a good start. How about joining some sort of equestrian club? There are some small towns here in South Florida we call "horse country" and in the parks in those towns, you will see people of all ages riding horses, either alone, or as a group. Try to find out if a park in your community has equestrian trails, and if they have organized outings. If so, join them, and if not, maybe the park ranger can help with community outreach to make people aware of these outings, so they can join you.
How about neighbors? Do you live in a farming sort of community? If so, you can make friends that way. Don't limit yourself to people who dress like you or have the same interests as you. I would have loved to have a friend like you, but I was the nerdy kid that people didn't talk to much. The few friends I had were the outcasts -- goths, metalheads, and the sort...not the kind of people you'd associate with a nerd who always wore her clothes tucked in with a belt, huh? These kids would have long hair, dark eyeliner, and dog collars. Well, we all had something in common -- we didn't fit in with the cheerleaders, the popular girls, the jocks or the rappers, and that is the point.
The quiet kid in your classroom who has trouble socializing may be the kind of quiet, intellectual, down to earth person you're looking for, and I bet that person would love to ride a horse or join you at the stables to help you take care of horses. I never owned horses but I had a friend in middle school when I lived abroad that did own a horse. It was tons of fun getting to pet the horses and sometimes getting to ride them (around the stable only, I was a newbie after all), though she rode alone on trails, or with her parents. I am glad she didn't assume that because I did not own a horse, I would have no interest in one. Most kids love animals anyway. If anything, that makes you unique, and a heck of a lot more interesting than the kid who has a TV in their room (don't most kids have that these days, anyway?).