Because of homeschooling, my daughter has been unaffected by peer pressure. Well, except when she made me buy her a New Kids on the Block comforter when she was six or so.
Unless she felt pressured to like that group because she'd feel rejected if she didn't, I don't see liking something faddish necessarily as a result of peer pressure.
Things become popular because they have a very broad appeal not because kids are pressured into liking them. The pressure comes afterwards when some (probably many) kids don't feel free to say they don't like something popular or feel there must be something wrong with them or they're missing something if they don't like something that "everyone" likes.
I was doing 5th and 6th grade Sunday school classes on Peace and Justice. We had discussed external peace and conflict and I was trying to get across the concept of internal peace and conflict so I asked them if they ever felt an internal struggle to act a certain way or pressured to like something because everyone else did. They all kind of looked baffled at each other and said no.
And it struck me that to these kids peer pressure wasn't about consciously burying themselves to fit in so much as learning the "right way to be". (Though I realize for some kids it is the more classic view of peer pressure, a miserable conscious process to either conform or be psychically abused.) It was as though they felt they and their peers were the sole source of the knowledge of How The World Works for each other. If they didn't go to school they wouldn't know the best way to dress or the best music or the right way to view the world.
So, do your kids have that peer pressure problem?
I think wanting to fit in with a group is inherent in being a social creature. How people respond to the need to fit in -- being a clown, conforming tastes to the group and so on -- may be partly learned, partly genetic. How strongly someone needs to fit in may be genetic for some kids and it may be a response to their home environment for other kids. Or a combination.
I think schooled kids are a special instance. School is designed to weaken the bond between parent and child so the child seeks a substitute for family at school.
(For those who haven't read John Taylor Gatto, that probably sounds like a conspiracy theory ;-) The Germans did want to create a society loyal to the state so designed a schooling system that strengthened the bond between the child and the state and weakened the bond between child and parent. The Americans who imported it just wanted an efficient system -- at least I'm hoping that's all they wanted! -- but that social part of the school system was inherent.)
So I see wanting to fit in to a group as different from the wanting acceptance by a substitute family that we see as peer pressure. Sort of the difference between being hungry and literally starving.