I could tell many stories about late bloomers. Have seen a lot of them over the years. Not just seniors in HS but several that developed in college. Kids that threw mid 80s as seniors and mid 90s a couple years later. Kids unrecruited and undrafted out of HS, only to attend junior college and end up pitching in the Big Leagues.
While the late bloomers do happen, it is much more ordinary to see the top underclassmen in HS, become the top seniors in HS.
Typically late bloomers are pitchers and most share certain traits. Tall and skinny, even tall and overweight, very good quick arm action, the ability to spin the baseball, etc., are all signs of future development.
Have also seen position players that blossom late in HS or in college. It's a little harder projecting them. But a good example would be Scott Schebler (Cincinnati Reds Outfielder)