As a student, doing well in school is your most important "job," and as Christians, we should be committed to doing the best we can at any job God has put in our lives. Colossians 3:23-24 says, "Whatever you do, work heartily, as for the Lord and not for men, knowing that from the Lord you will receive the inheritance as your reward. You are serving the Lord Christ."
You are not only trying to get good grades in order to do well and have a nice transcript for college (and maybe monetary rewards), but you are also glorifying God by being committed to doing the best you can. This is a part of your "quiet" testimony, living your life in a way that Christ would (Matthew 5:16). Good work is what God did when He created the universe, and He said it was good (Genesis 1:31). Our work should be good too!
Ephesians 6:7-8 says that we should "serve wholeheartedly, as if you were serving the Lord, not men, because you know that the Lord will reward everyone for whatever good he does, whether he is slave or free." We do our best at school, work, or whatever because God has entrusted us with gifts and talents that we should be using wisely.
1 Corinthians 10:31 tells us, "So, whether you eat or drink, or whatever you do, do all to the glory of God." Going to school and being a good student is definitely one way to give glory to God with your life. Try to be the best student your teachers have ever had by studying hard, being kind and respectful to everyone at school, and doing good work on your assignments, papers, and speeches.
All that said, it is NOT a sin to get bad grades (or even just mediocre ones). Your letter grade is neither sinful nor unsinful. It's a neutral thing. Sin occurs in our hearts, our motivations, and our attitudes—not in the actual thing itself.
For example, if you are truly trying your hardest at school and seeking help when you don't understand, but you are still struggling to make As or Bs or even a C, that's OK. It is not a sin. Do try to find a tutor or someone else who can help you though, because it is definitely frustrating and distracting to have a hard time learning things at school. That frustration can lead to sinful actions like anger toward others, resentment, bitterness, etc.
However, if you are slacking off and the result is a report of low grades, then you are not "doing all for the glory of God," and that IS a sin. Your low grades would be a natural consequence of your actions though—not a sin. The sin is in the attitude of not caring about whether or not you do your best.
According to these verses, the Bible is saying that a strong work ethic is important and that slacking off is not a good idea for the believer.
That said, it's also important to note that Bible warns against making "work" all that matters to you (Matthew 6:19-34). Being obsessed over getting good grades or winning awards at school isn't good for us either; it means that our priorities are out of whack. That kind of misplaced dedication can cause health problems with stress and depression if we don't meet our goals. It may even build a wall between you and God, your family, or friends.
Yes, do your best, but don't overdo it to the point that you hurt your health or relationships. God rested on the seventh day of creation (Genesis 2:2-3), and later He commanded that His people take a Sabbath too (Exodus 20:9-10a). Today, because of the grace of Christ, we are no longer required to take a Sabbath (Mark 2:27), but it's definitely a wonderful gift that it would be wise to use!
Remember, while good grades are important, they should not be the most important thing in your life. Your heart, attitude, and motivations are what God is concerned about, and His deepest desire is to help you develop into Christ-likeness (Matthew 6:19-34). God is interested in having a relationship with you as His child—not hounding you about the letters on your report card.
No, it's not a sin to get bad grades (or even mediocre ones), but you still need to do your best. Ecclesiastes 9:10 says, "Whatever your hand finds to do, do it with all your might." Also, being a good student is being an awesome example of a hard-working Christ-follower (Colossians 3:23-24). That said, don't overdo it either! Grades are never more important than your health or relationships (Matthew 6:19-34).
Cat is the web producer and editor of 412teens.org. She loves audiobooks, feeding the people she cares about, and using Christmas lights to illuminate a room. When Catiana is not writing, cooking, or drawing, she enjoys spending time with her two kids, four socially-awkward cats, and her amazing friend-amily.