Calculus, in particular, seems to be frequently debated with respect to whether it ought or not to be included in the high school syllabus
Just to clarify, I think that calculus should be in the high school syllabus. But if you want to properly learn math, it's better to introduce calculus after covering more basic math at a deep level. This may be impractical for a school syllabus but will be really important if you want to gain a solid foundation of math which will allow you to be able to properly get to more advanced math later on (for math, statistics, physics, etc.). Geometry, trigonometry, permutations and combinations, some basic number theory, etc. are things I would suggest working on first. Calculus is not such a big deal as such for a gifted math student in high school, but I see people trying to complete the "sequence" quicker, with multivariable calculus and linear algebra in high school, and wonder what the point of it is.
Edit: I understand that the college credits transfer and it's a good idea from a college admissions perspective. However, what they teach at high school will be so much less rigorous than it should be at a good university anyway, so you will probably need to relearn a lot of it. The better your skill at problem solving and conceptualization, the better you'll fare once you get to the actual hard stuff.
