The basic categories of players eligible to be drafted for the MLB are:

- High school players, if they have graduated from high school and have not yet attended college or junior college;
- College players, from four-year colleges who have either completed their junior or senior years or are at least 21 years old; and
- Junior college players, regardless of how many years of school they have completed.

http://mlb.mlb.com/mlb/draftday/rules.jsp

This would put an end to the "one and done" situation.