I'm going into my 2nd year of university and trust me when I saw grade 11 is the PERFECT time to take her out of school! It's the last year you'll ever be able to go take time off school for a vacation.
Grade 12 is really vital to not miss much, and you'll never be able to take time off in college/university.
I wouldn't have had a huge problem taking time off in grade 11, just talk to her teachers, make sure she's not going to miss any REALLY important tests/assignments, and if she is find out how she can make them up.
It shouldn't be too hard for her to catch up.
And TRUST ME! She'll thank you, because a week away from HS life to be in DISNEY would've been a God send for me!!