I guess one of my questions is precisely that. If some of these teens are rebelling to get to where they are, will the beliefs carry over into adulthood, or be shed like something unfashionable at a later date?
They already have a word for that: you’re a HIPPIE!
Ahem I’m thirty. I’m too young to be a Hippie, as all the old grizzled Hippies constantly remind me. I’m a NEOhippie, thankyouverymuch! 
'Sides, Hippie is not a religious designation and never was. There’s lots of hippies who are not neopagans, and lots of neopagans who are very decidedly not hippies. I happen to be both.
Back to the OP: I was also a Christianity seeker in high school, and dropped it almost exactly when I graduated. For me, it was part of a searching for a group of people who had the some ideas about the universe and God that I did. I spent a great deal of time trying to convince myself that I didn’t have to agree with everything a church said to be a member, but came to realize I didn’t even agree with 40% of what any churches said. This realization that what I believed was more important than fitting in came about when I started college. Soon after, I found out that what I believe DID have a label and groups of people who also believed it: neopaganism. So I finally found my group after all, but only after I stopped looking. So, yeah, it’s very possible that some, if not most, of these kids will drop it when they get to college. Maybe not. Only time will tell if they’re true believers or trying to fit in - and if trying to fit in trumps their personal beliefs or not. For me, it didn’t. For others, the social connection might be more valuable than their personal truth.