I was a A/B student in high school (graduated with ~3.5 cumulative GPA). In college I really slacked off during the first few years, but did well the last two (again, graduated with ~3.5 cumulative GPA). In medical school, I have done very well (GPA ~3.95, AOA, 250/270 Step 1/2 scores).
It is truly up to you when it comes to excelling in school. The more time you commit to learning, the better you will do. It's simple as that. It might not be the sexiest answer, but it's the truth.