Hi and welcome!
My daughter was diagnosed at 16 too. I'm guessing that they have checked for infections? That's one of the other things that can raise fecal calprotectin.
I hope the scopes give you some answers. The prep is the worst part of the scope - the actual procedure is usually pretty easy - my daughter calls it "a good nap." Having something to watch or read on prep day helps, something she can take into the bathroom with her, because she'll be spending a lot of time there! Wipes (flushable) will also help. If she is given the Miralax prep, she'll have to mix it with Pedialyte or Gatorade and will have to drink quite a lot.
Good luck - let us know how it goes. If she is diagnosed with Crohn's or Ulcerative Colitis, there is a lot of great info on the forum. Hang in there!