I think the answer to when does school become challenging is soooo so very dependent on school culture and ultimately individual teachers. My kids have been challenged and then-again not challenged at the *same* school but with different teachers.
We've also had our kids in a school that was very proud of it's elective and choice classes, and honestly, the classes weren't all that great - my kids enjoyed core classes that were well taught much better than electives that were supposed to stretch their minds because they offered "interesting" subjects that were not normally a part of the curriculum - the key with the electives was almost always the relative enthusiasm of the teacher. Hopefully that won't be an issue with your dd's school!
I tried to never build up my children's expectations about school too much - we have switched schools to have better academics and a higher level of challenge, but we kept the explanation at that. We didn't tell them that things were guaranteed to be a whole lot better, just that we were working (always) in that direction.
And... fwiw... one thing that my children did really notice and appreciate was that the overall classroom atmosphere was less boring for them when they able to be in classes that were supposed to be gifted and/or challenging - even if the classes were still not challenging (or even outright boring) - the lack of the distractions of slow and easy questions from other students made a huge impact on how much they enjoyed class.
Best wishes,
polarbear
