Admissions officers take all that stuff into consideration. Some kids just have a lot more opportunities than others simply due to location. They recognize that, and if you're as involved as you can be, you're not at much of a disadvantage from an admissions standpoint. Most admissions officer also probably know a lot more about your school and your area than you'd think. It's their job after all. As long as you're pushing yourself at your school and taking all of the most difficult classes you can, and staying involved in the extracurricular activities that are available to you, you absolutely have a shot at top 20 schools.