Graduation rates tend to correlate with selectivity. A school that brings in stronger students will have higher graduation rates. Your chance of graduating in 4 or 6 years is probably not much different between your reach school and your safety school.
Note that, for pre-med purposes, if you skip any pre-med courses with AP or IB credit, most medical schools expect you to substitute more advanced college courses in those subjects. E.g. if you skip calculus 1 with AP or IB credit, a medical school that wants to see calculus will likely want to see calculus 2 or 3 in lieu of calculus 1.