I got really bad grades. How do I fix that even though the year is over?

Okay so I have always gotten good grades. "A" and "B", but the last semester they mail out the report cards and my mom just got mine. She was pretty mad . D's were tered all over it.(I got such good grades I thought it had become extremely easy so then I started to slack and I thought school was way too easy when I realized no it isn't!) Blah. But I can't make that up because the year is already over!!! So now what do I do? I just finished 7th grade and I want to get into a good high school and college. I would have signed up for summer school the only thing is I had no idea my grades were so bad. And the summer is already started and it is way too late to sign up now. So what do I do? Repeat the 7th grade? But then I would be extremely tall and tower over everyone, for somereason all the new 7th graders are such shorties! Help!