In SC, it's illegal to carry into a school. However (correct me if I'm wrong) the state has decided a parking area is public and thusly legal for your weapon. Just leave it in the car when you go inside. Some private companies have "no weapons on premises" restrictions. That is fully their right as a private property. In that case, unless you want to leave your weapon at home, it's "what they don't won't hurt ya."
I suggest you read your state's firearms laws thoroughly. Good luck, sometimes the wording can be ah...confusing.