I know your efforts may seem useless–but how do you know they really are? Study after study shows that most people make their commitment to Christ while they're still young–many in their early teens.
After all, God has promised that when we teach His Word, our efforts are never in vain. This is because His Spirit is at work, and He promises to bring forth fruit. If you are teaching the Bible, God will use it to touch lives, even if you can't see it. The Bible says, "My word that goes out from my mouth ... will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it" (Isaiah 55:11).
Do you remember Jesus' parable of the seed and the sower? (You can find it in Luke 8:4-15.) Jesus said that when we teach God's Word, we are like a farmer sowing seed. Not all of it takes root and flourishes–but some of it does: "It came up and yielded a crop, a hundred times more than was sown" (Luke 8:8).
Make it your goal to be the best teacher you can possibly be. Make the Bible your textbook, and work hard to make your lessons clear and interesting. Involve the class in discussion, too. In addition, pray for them, and ask God to make them open to His Word. Challenge them also to give their lives to Christ. And remember: Your example of faith and love is just as important as your words.