I commend you for displaying Christian love and concern for your neighbor. As you mentioned in your question - odds are good that this girl isn't going to be receptive to you trying to help her, but it is important that you attempt to keep her safe spiritually and physically. You are doing the right thing. Even though it is scary, I would encourage you to try and talk to this girl privately. She might be more willingly to listen to your concerns in that setting. If you can honestly do this, you might also assure her that you haven't talked to anyone else about this. Hopefully this will keep her from getting defensive right away. Finally, don't accuse her of doing wrong. While unlikely, it is possible that she is not guilty of any sinful behavior when it comes to these boyfriends. Instead of accusing, take the concerned approach with her. Maybe start out your conversation by saying, "I'm not accusing you of wrong doing, but as a fellow Christian I am concerned about you because of some of your recent behavior... 1. Meeting boyfriends online 2. Getting together with them at your home without your parents being around. Do your parents know about this? Bring those concerns up and see what she says. She might confess some sins to you that would present an opportunity for you to offer her the comfort and forgiveness of Jesus.

Then again, maybe she will deny that she has done anything wrong. If that is the case, then in Christian love - remind her who she is... 1. A blood bought child of God through faith in Jesus. The Savior that opened the door to eternal life in heaven to sinners like us. 2. A Child of God who has been created to give honor and glory to God in her thoughts, words, and actions. 3. A loved child of her parents. Would you continue to do the same things if your parents knew about them? In Christian love also warn her about the dangers of her actions, even if she is telling you the truth that she has not sinned with these boys. Warn her about what her actions might lead to... 1. Internet predators - that some grown men pretend to be 14 year old boys so that they can meet teens for sex. 2. Even if the people she is meeting online are her age, remind her that these boys might be looking for the same thing that the grown men pretending to be 14 years old are. 3. Sexually Transmitted Diseases and Pregnancy. 4. Emotional and Physical Pain 5. God tells us certain blessings (companionship, sex, kids) are to be enjoyed within marriage. He sets these guidelines for our good. You can't hide anything from God. Do you want your behavior to affect your relationship with Him?

If your private talk with her doesn't work, then I would discuss this situation with your parents and/or pastor. As a parent, I certainly would want to know if something like this was going on with my child. Be prepared for some unfair treatment by this girl and possibly other kids your age. I realize that isn't pleasant or fun to deal with, but it is part of bearing our crosses as followers of Jesus. You are doing the right thing and every adult would support and commend you for your Christian concern. And your classmates and this girl might surprise you with their reaction as well.

Be sure to bring everything to the Lord in prayer.  He will be with you when you talk with your friend, and he will guide your words and actions, and hopefuly guide the words and actions of your friend through your encouragement and concern.  God be with you!