The best story I've heard in this vein is Justin Verlander, who pitched at Goochland HS in Virginia. Goochland is only about 23 miles from where my son went to high school, but in a much more sparsely populated area, and so they played against smaller schools and not ours. Plus he was 7 years older than my son, so for those reasons I never actually knew of Verlander when he was local and never saw him in person.
What I've been told is that he was stick thin coming up. There were lots of people who talked about how hard he threw, but that was largely based on being upper 80's as a junior. His senior year, he had mono or something like that, and his velo capped out around 89. He had committed to ODU but was interested in pro offers. The tale told to me was that he was ready to sign for 200k but MLB teams weren't ready to pay that for where he was in June 2001.
Three years later, he was healthy as a horse and much larger and stronger. I remember he came to Richmond to go head to head with Justin Orenduff, a low/mid 90's sinker baller at VCU. The local paper covered it and reported with amazement on the number of pro scouts and RADAR guns in evidence. Verlander topped 100 at times that day. Not long after, he was the # 2 overall pick by the Tigers.
And the rest, as they say, is history. (And much more verifiable, I might add!)
BTW we don't get to see him around here much any more. He built his parents a mansion overlooking the James River, and he visits them on occasion. Last time around, I think Kate Upton got more attention than he did!