You've probably seen kids who've gone Goth. They are the ones who dress all in black, tend to wear black makeup, and they wear lots of crosses. Many of them will wear makeup to make their skin look pale. Basically they are trying to look like vampires and this really upsets their parents.
The typical Goth kid is just exploring it as part of finding or developing an identity. All kids go through such a development stage in one way or another. Goth is just one of the paths. Most will eventually grow out of it and be "normal people".
For those who don't grow out of it there may be some serious implications. A thirty year old Goth will find it hard to remain gainfully employed. Some will, or have, turn to drugs to "enhance" their "undead" experience. The worst part is that these people will still crave social interaction. This drives them to interact with the Goth kids.
Many Goth kids are victim-wannabes. They long for some incident that will suddenly give their lives meaning and drive. They also want to be accepted by a group of peers who "understands" them.
Some, of course, have already had bad things happen to them, such as sexual abuse. In these cases the kid has no hope or sense of self worth. They also tend to feel powerless. The fantasy of being magically turned into a powerful predator, such as a vampire, is very appealing.
When a thirty-something Goth loser builds a following of teens, it is rarely for the good of the teens. The kids will be encouraged to stay Goth rather than grow out of it so the loser won't lose his social group. They may be given drugs or introduced to sexual activity for which they are not prepared. The old guy will do what he can to keep the group together because it validates his existence, whereas in regular society he is just a loser. Keep in mind that when I say "guy" here it could just as easily be a female.
On some occasions, the older Goth loser was the victim of abuse and is still lost in the fantasy. When this happens, they will often try to get the younger Goths to engage in sex at a younger age to "prove" that such things are "right". If it is all right and normal for these kids to be engaged in sex, then it was all right for the older Goth to have been molested as a kid. Like with many forms of abuse, it is a pattern that will repeat.
In general, the Goth phase is not harmful to kids. Naturally, once drugs or sex are introduced the harm increases, but these things are just as (if not more) likely in the football/cheerleader group. It's just part of being a teenager. As with all things involving kids, parental love, respect, trust, and communication make up the key to raising a good kid.
Please note that all of the above is a generalization based on my own observations and interactions with various peoples. I am not an expert in sociology, psychology, or child raising. If you have concerns about your children, please talk to them and their school counselors. If you have reason to believe that your child is doing more than "going through a phase" then you need to seek the help of a qualified child behavior professional, though probably not a religious leader unless that person is trained in modern psychological methods.