The only thing high school grades are good for is to get you into college, and to make you eligible for scholarships to help reduce the debt that you will be sinking yourself into. Grades also give an indication about your work habits and work ethics that you are developing. If you have taken difficult classes and have shown that you can balance the work load with the rest of your life, then you know a little about how to manage your time effectively.
What IS important once you get to college is your willingness to take that fresh start in college and change any bad habits that you may have developed in high school. Like "I can do that later, I want to play video games right now." Or "the teacher in high school let me turn papers in late or re-take a test, the professors in college will do the same (they probably will not!)"
Some people that do bad in high school are because they don't care, so they don't try. Some are because they are lazy. Some are because they are unable to understand (translate into don't try hard enough to understand).
Which are you? Apathetic, lazy, or unintelligent?
Apathetic: with a goal in mind, this type can force themselves into changing their study habits and do well enough. They can become enthusiastic and perform at their highest level.
Lazy: more likely than not this type will slack off again and be back on here saying "oh noz! I failed gen chem" how will this affect my getting into med school!!!!" Eventually this type will become a business major or pre-law, or something else that does not require the dedication that it takes to get into med school.
Unintelligent: well, there are degrees and occupations that will work for people like this... They find their happy spot, it just is not in medicine.
You can do it, if you want it enough, try hard enough, and are good enough.
dsoz