In our district, junior year is the toughest in terms of workload.
I hope your daughter is not focused entirely on grades, AP and percentiles. The kid next to us was "owned" by AP pressure, and he had no high school life. Everything was "hard" and there was no time for a social life or activities. My son, by contrast, didn't take AP classes except for one during his senior year. He had several activities (track/XC at school, stuff through the community, some things through the synagogue) and leadership development opportunities. He didn't stress about test prep courses, and he took the SAT and the ACT once each. He got into a better college than the kid who was miserable and had better grades.
My son also had a couple of schools on his list. He had not decided that there was only one school for him, as your daughter seems to have done. I think that's a huge risk, frankly. Also, remember that a highly competitive school like her choice will have an entire freshman class of high performing high school grads, and she will be in classes with competitive kids with high grades. If she cannot handle the workload now, in 11th grade, how will she handle that and much more in college? How, why and when did she choose this one college? Is it possible that her perspective may expand and change throughout this year and into senior year when she has to decide?
From your brief post, it really sounds like she's not talking the long view here. There are plenty of great colleges for every student, and it's not just about grades and test scores.
Yes, she should talk to her guidance counselor and maybe she should visit the school she is hell-bent on and talk to the admissions staff.