I'd lean more to B if she'd be repeating the same books.
Not that I don't re-read books but there's so much good literature for her still to meet.
There was a third option, option C, that allowed her to go to 7th grade reading. because of the scheduling, she'd also have to do all her art, PE, music, etc with 7th graders. It caused all sorts of scheduling problems and I didn't like the massive moving around it required. No one in the room, we parents included, liked that one. The school has offered the reading and library specialists to work with her reading teacher (standard and gifted) to provide different books along the same genre. DD loves to read, read, read, so to me, this will be her "easy A" class.
I wish I could attach the optins as written by the school. I think you'd be impressed in how thorough they are.
Based on all the discussions, I think 6th grade is going to be a challenge in all areas.
