Looks like I missed the conversation about grades in school, but wanted to put my 2¢ into the mix.
Grades are very important! For me, good grades put free scholarship money in my pockets. This scholarship money allowed me to get through five years of college (yes, I changed majors, a little more drastically than most, starting in art, going through music and architecture, and ending up in electrical engineering) without a single loan to pay off after earning my degree.
I graduated tenth in my high school class with a 4.02 GPA. I, like many, didn't even know what a GPA was until my junior year when college was bantered back and forth. I only knew that I was capable of getting A's and I typically got them. Except for Mrs. Marsello's AP English classes in tenth and eleventh grades - I think she had something against me.
So, fast forward to college. I was lazy and rarely studied. I got through EE with a 3.27 GPA. For each of the 118 resumes I sent out over the three months following my graduation, I had to attach a transcript. If those transcripts showed a 2.3 GPA, I wouldn't have had any considerations from any prospective employer. Why do I know that? Because I am now a hiring manager, and when I see a shit GPA come across my desk, regardless of any projects that person may have worked on, the resume goes to the bottom of the stack. If that person gets lucky and I weed through all the resumes on top of that shit GPA, they may get a call back. Typically, they don't.
The "real world" is the largest competition you will ever enter in your life. You should take all the tools, credentials, knowledge, and initiative into it you can muster. When getting your first job in your field, all you have to rely on is what's listed on a piece of paper (unless you have an inside track like an internship or co-op or friendly uncle CEO) and what's on that paper had better catch the reader's attention; otherwise, say hello to McDonald's.