If it's not God's will for you to continue teaching this group, then you should quit. But don't stop unless you know beyond a shadow of a doubt that He wants you to!
After all, you don't know what's really going on in the hearts and minds of the young people in your class. On the surface, they may appear uninterested but God may be speaking to some of them in ways you'll never realize this side of eternity. Jesus said that teaching God's Word is like sowing seed but seeds don't become full-grown plants overnight, and there won't be any plants at all if the seeds are never sown (see Luke 8:1-15).
Remember: There is power in the Word of God, and when you teach, another Teacher is also at work: the Holy Spirit. God's promise is for you: "My word ... will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it" (Isaiah 55:11). Are you praying for your students, and are you asking God to work in their hearts by His Spirit?
Make sure the Bible is the foundation and center of your teaching. Make sure also that you are as prepared as possible, not just understanding each lesson but thinking of ways you can make it interesting. Involve the students in discussion by asking them questions from the lesson and most of all, challenge them to commit their lives to Jesus Christ.