Until my son was in about 7th grade, it seemed that all of his friends were sweet innocent kids. About half way through his 7th grade year though I started noticing a change in my son that I didn't like. His closest friends at church and school were good kids but there were a few that he had started to spend time with whose influence was not so good. These kids were nice and sweet and we liked them just fine but they were not friends that encouraged our son in his beliefs as a Christian or ones that live with the same rules, morals and principles that we teach our children.
That being said, even though we have taught our children right from wrong and how to behave, etc... doesn't always mean they are going to do everything the way we would like. We live in a fallen world where bad choices are around every corner and friends that do not have the best of intentions with our children are around those corners too. Our children will have to make the choice and hopefully it will be the right decision. For my son, a few times he chose poorly and he has been corrected, punished and prayed for over these matters.
The Bible says in I Corinthians 15:33 that bad company corrupts good character.
33 Do not be so deceived and misled! Evil companionships (communion, associations) corrupt and deprave good manners and morals and character.
Since I began praying about these children and my sons association with them One has moved, One seems to have lost interest in my son, One is at a different school and we are still waiting to see what God does with another. :)
God is so good. He cares about everything and will help us with it if we will just ask.
On the role model end of this, remember you are your child's biggest and most important influence. Live the life in front of them. Read your Bible, pray, go to church, tell the truth, be careful of what you say and watch on television, stand up for what is right, use your manners, be kind and so on and let them see that in you.
From the Book
(The following are quotes and passages from Stormie Omartian in her book "The Power of a Praying Parent")
We can pray for friends to either be changed or removed from our children's lives.
Parents often have gut-level feelings about their children's friends. When that happens, ask God for Holy Spirit-inspired discernment and pray accordingly.
2 Corinthians 6:14-15 instructs us not to unequally yoked with unbelievers. That doesn't mean our children can never have an unbelieving friend. But there is clear implication that their closest friends, the ones to whom they have strong ties, should be believers.
If your child doesn't have close believing friends, begin to pray right now toward that end.
Do not stand for someone leading your child astray.
Pray that God would take anyone who is not a godly influence out of his (her) life or else transform that person into HIS likeness.