Bryce Harper finished high school early so he could get a season of juco in before being drafted at 17. Trevor Bauer finished high school early so he could get three seasons at UCLA before being drafted at 20. If you actually have a shot at playing professionally you want to be accelerating your development not delaying it. And if you don't have a shot at playing professionally, what is the point of repeating grades. MLB teams have zero interest in 25-year-old seniors.