That was my daughter at that age. When she was in 5th or 6th grade she was writing fan fiction novels and posting them various places and getting lots of reads. Also winning various creative writing awards.
Fast forward. She’s now a HS junior and has been college shopping in advance of applying to college in the fall. In the intervening 4-5 years she has gone through a writing phase, a digital arts and animation phase (was creating her own animated shorts) and is now thinking of studying both molecular biology and digital arts/animation in college. She also plays 3 instruments. She is looking at a variety of liberal arts colleges and still interested in writing, but wants a well-rounded education and wants to dabble and experiment with a lot of things.
Don’t be in a rush to narrow her down at age 13. Most good liberal arts schools have good writing programs. Writing is at the core of a good liberal arts education. I’d be looking for schools that give her a good well-rounded liberal arts education as well as room to write.