However, if she has finished AP level maths by the end of the sophomore year does that mean she will not be able to 'graduate' high school because she will not have the required years of high school Maths and therefore not meet matriculation requirements for colleges here?
I had been thinking that with Maths (up to Calculus BC) under her belt she would be free to delve more deeply into other interests if she still had high school years to go before university because CC's here are not great.
Am I wrong to after school Maths after all?
Yes and NO.. For math you can do this easier than English. And it depends on the school. To get into my local university one only needs to prove at least mastery of Algebra II. One can take the SAT II Math Test for this, or pass AP Calculus Test. (NO class is required to take the test.) Officially our school requires 2 years of math, at minimum passing Algebra to graduate. But you must take 2 years in H.S. But if you take A.P. Calculus as a freshman (as one of my son's classmates has done) I believe your math requirement is considered done as there are no higher math classes.
My son just told me that other freshman in his H. Algebra II class are taking the AP Calculus test tomorrow. Not sure what that accomplishes, perhaps they won't have to take any more math at the H.S.? In my district the math co-coordinator is strict and if you want to take the honors math sequence you MUST take it in order from the district and not skip. He does not allow any outside coursework to count. (He is not popular amount the gifted kids parents.) On the other hand it is possible to get on a track to take Calculus as early as a 9th grader but you have to start back in 5th grade.
On the other hand. English requires 4 years in class. You can take AP English classes, and you might be able to get a summer or online English course to count. But you can't just test out of it. And 3 years of Social Studies.
Foreign Language is the only other course that I know of you can test out of instead of sitting through a course. My son mentioned that a number of kids he knows are taking the AP Chinese test this spring. These are kids who speak Chinese since birth and have been taking Saturday school for years. Universities will usually count an AP Test or a SAT II test. For languages the SAT II test can be harder than the AP, I was looking into this because my son was interested in taking Japanese a class not offered at his school. My H.S. only required 1 year of a foreign language OR 1 year of performing arts so this isn't a big deal for H.S. graduation.
There are ways particularly in math. Some school districts have higher level math. Some private schools offer more flexibility. My district does accept online courses and classes taken at a community college or university to count. But only one or two of those courses per year will count towards graduation.
Good Luck. Keep in mind that what is required to GRADUATE from H.S. in the U.S. is different from what is required to get into university. Universities can be more open to accepting a test as mastery, and many U.S. high schools have very low requirements.