The freshman-to-senior gain is about a second
Nobody publishes a real grade-by-grade dataset for high school sprinters, but the varsity ranges above put a typical boy around 12.3 as a freshman and 11.4 as a senior, a gain of roughly a second. Some of that is training and some of it is just turning 18. If you are ahead of that curve at 15, the later gains get harder, because you have already banked the free speed that growth hands out. The same climb over barriers is charted in the 110m hurdle times by grade.
