OP, some kids in the USA take courses outside of school to accelerate their math curriculum. For example, while the normal progression for math might be to take Algebra I as an 8th or 9th grader, a student could take an intensive algebra course over the summer after 5th or 6th grade and then take geometry and/or Algebra II as a 6th or 7th grader. In turn, that means he’d be ready for calculus in 10 or 11th grade, provided he follows the sequence normally after that. Also, some schools here provide an accelerated track for the top students that has them ready for calculus as a junior. As a result, they can then have room for Physics BC and AP Stat etc.