^ Actually, there are two types of calculus sequences:
Those where AB and BC are two levels of Calculus, parrelling Calc 1+ Calc 2 in college, each lasting one year.Students who take Precalculus as juniors take AB as seniors, students who take precalculus as sophomores take AB as juniors and BC as seniors (if they wish, as they can also take another math after AB).
And
Those where AB and BC are two “intensities” of Calculus, where AB covers Calc 1 in one year, and BC covers both Calc 1 and Calc2 in one year (ie., college-paced vs. AP paced). Regardless of what year students took precalculus, they have to choose either AB OR BC.