You don't have to give up, but you do need to do better. Medical school certainly isn't going to be easier than undergrad, and schools want to know you'll be able to suceed. I understand the sentiment of coming from a poor high school, I did as well. It can make things hard, but you have to take responsibility for your own grades. Get extra help if you need it. Finally, schools aren't going to take "I go to a hard school" as an excuse for poor grades, so don't try to use it as one.