I have found it sad seeing how much pressure is put on our young people in school these days. Bad grades DO NOT mean your life is doomed. The important thing is to learn from both your successes and failures. Use the bad grades to figure out your weaknesses and strengths. Are there certain subjects you do not enjoy? (Don't study them anymore after you complete high school unless they are just a minimal prerequisite.) Do you need to put more systems in place to cope with your weaknesses. (I can be absentminded {ADHD?} so I have always constantly made lists, used a calendar, hung my car keys on a peg next to my door for more than 35 years, etc.) And I DO NOT think everyone needs a standard 4 year degree. My mom only took a few college courses and she had more brains and common sense than my dad, brother, sister and I (we all have advanced degrees--more than just bachelor's degrees). My dad dropped his good paying job in the city when I was in elementary school to move to the mountains and become an entrepreneur, it was my mom's business instincts that made them a success. My dad still talks about the specific "moves" that she pushed him to do (in business contracts and such) that made the difference between success and failure.
I know telling your parents will be hard and I hope they don't overeact but some bad grades might even help you in the long run. Sometimes when everything is easy, we don't become as aware of how to work to overcome things in our lives. Life has a lot of ups and downs. It is important to figure out how to come back after you are down. These bad grades may help you learn this lesson. I know school is not easy! Please try not to panic (I panic all the time) and hang in there.