This is going to sound unpopular given the audience here, but I'd bet most examples of "X" in this context will be corners of math. Math tends to be more cumula

This is going to sound unpopular given the audience here, but I'd bet most examples of "X" in this context will be corners of math. Math tends to be more cumulative than most other subjects taught in high school, and as such it's the subject that jumps to mind when the answer "next year" makes sense when someone asks "when am I going to use X?" Trigonometry, for example, or matrix math are highly esoteric in the context of a usual high-school student, yet they show up all over the damned place when you get out of high school and into more advanced studies. This is a real shame because math is essentially a vocabulary for solving problems and there should be dozens of answers a teacher could give to that ? besides "next year."