No, it wouldn't be wrong for you to turn it down -- if God doesn't want you to accept it. Someone else may be better qualified, and God may want them to accept this responsibility instead of you.
But what if God wants you to take this offer? Then it would be wrong for you to turn it down, and you should seriously consider this possibility. In other words, pray and seek God's will about this, and don't turn it down just because you don't feel comfortable or qualified to do it. If it's God's will, He'll help you and use you, as you depend on Him. The Bible says, "'Not by might nor by power, but by my Spirit,' says the Lord" (Zechariah 4:6).
Study after study confirms that most people make their decision for Christ before the age of 25 -- and often during their early teen years. If you do take this class, don't think of it as a burden but as a privilege, and as an opportunity to influence them for Christ for the rest of their lives.
In addition, be the best teacher you can possibly be. Prepare carefully... pray incessantly... and present the material interestingly and creatively. Make the Bible the foundation of your teaching -- and most of all, keep Christ at the center. Pray too that they won't only hear about Christ through your words, but they'll see Him through your life. The Bible says, "Be shepherds of God's flock that is under your care" (1 Peter 5:2).