Friends are one of the most important parts of your teen's life, and while your teens aren't rejecting you, they may be reorganizing the amount of influence that you have on their opinions. If you have a good relationship with your teens, you may find that they do accept your values concerning the important issues of life (religion, honesty, how to treat others), while taking on the opinions of the peer group in terms of dress, music or hairstyle -- superficial accoutrements that are more visible but less important than values. While the peer group may influence your teen, be aware that sometimes the influence can be a positive one, if it motivates your teen to do well in school, become involved with the community, or exercise.
Realize that the group that your teens hang out with may be very similar to them, as teens gravitate to those who are like them and who share common interests. If your teen is drawn to a group that cuts classes at school, say, take a good look at the factors that are attracting him or her to it and see if there are any actions you can take to get to the root of the problem, such as arranging for a tutor to get your teen caught up with schoolwork. Teens who don't have friends can be worse off, as they may suffer loneliness, unhappiness and low self-esteem. They may drop out of school, become involved in criminal behavior, or join a gang as a social outlet.
What you can do as a parent is to build up your teens' self-esteem so they'll be less susceptible to negative peer pressure, and role-play situations in which they may want to decline to take part in an activity while still maintaining the friendship. Make your house a welcoming place for your teens' friends to congregate, provide refreshments, or have your teen invite a friend to come along on a family outing. Encourage positive teenage friendships and don't be jealous that your teens prefer to spend so much time with their friends, as discouraging a negative friendship is much trickier.