I know a pitcher who was just average in high school and on his local travel team. But, he was very good by his senior year of high school and had several good outings for his showcase team between his junior and senior years of high school Consistently threw well in bullpens with a high 80's fastball and a decent curve.
Went to a local D2 and the coaching staff changed up some things with his delivery and by the spring of his freshman year he was hitting 93 on his fastball in games and added a sinker and a slider. Had some dominant outings in his freshman season including a 1 hitter. Transferred to a JUCO to become draft eligible after his sophomore season. Didn't perform well at the juco and suffered an arm injury. Is back at the D2 and hasn't pitched due to arm problems and surgery. I doubt he will pitch again.
Kind of an odd example, but he did blossom from an average high schooler, to a very good pitcher by his senior year of HS, and to a very good collegiate pitcher as a freshman, even though it was just for one year.