Advice on what to say and how to talk to my mom about my report card?

I'm a sopre in high school and this past semester was brutal for me.. I got tons of C's and I even got a D and F on my midterms . I'm usually a straight A student, up until this year, and my mom told me that if I EVER got a C on my report card, then I'm walking myself to school (4 miles) and back, and all that nonsense. This time I got an F... It's supposed to come home tomorrow and I just don't know what to say to my mom, as she's not really the understanding type. She'll just yell and lecture me for months, literally and she doesn't understand that no matter how smart you are, school gets really tough sometimes. Any advice?