Before you make a final decision, check with your state educational agency as well as the central office of your school district to get a definitive list of graduation requirements and you may discover that there is no need to accelerate your DD for a semester unless you are trying to save your district money by removing the obligation to provide services earlier than necessary. That age 21 requirement is only for certificate kids, not diploma kids. If I had to do it over, I would not have allowed my oldest DS to qualify to graduate at 18 as he would have benefited from services until age 21 and there is a dearth of services for youths in that 18-21 age range in my state.
Similarly to Spaghetti, our state/district only requires about 22 credits to graduate and students typically earn 8 credits per year. Most students can graduate in 3 years if they double up on English one of the years and social studies one of the semesters. Since last year, high school students are supposed to take math every year of high school but if you are graduating a year early and have at least Algebra I, Geometry and Algebra II, you would be okay anyhow. Unlike in your district, most of the students in our district also enter high school with at least 1 or 2 high school credits due to Algebra I and/or a year or two of a foreign language in middle school. Most GT tracked students enter high school with 3-5 credits. DS entered with 8 and will probably graduate high school with double the credits "needed" after including dual enrollment courses. However, a student cannot enter high school with credit for an "AP" course on his transcript prior to high school due to CB limitations although that student can certainly ace the AP exam and argue for placement.
If your DD intends to continue to college, then you want to make her as prepared as possible, which means more education, not less, prior to high school graduation.
