One school I know of, University of Pittsburgh, has an early admission program to its graduate schools (law, business, medicine, and others) for qualified juniors. That is, students can apply to the program in the second semester of their junior year, and if admitted, can use the first year of the graduate program as their "senior year," in terms of credits needed for their bachelor's degree.
More to the point, this school has "guaranteed" graduate admissions for outstanding high school students, to which one of my smartest high school classmates was admitted. She had outstanding grades (4.0) and board scores (1500), and was "guaranteed" a place in the medical school upon matriculation. The minimum requirement was that she had to take a "general science" major (biology, chemistry, physics), and earn a 3.5 GPA, but given her high school achievements, this was not in doubt. (She now has a thriving practice.)
For someone that knows what they want, are there any meaningful downsides to "enrolling" in a graduate program (medicine, law, business), right out of high school?