There's a lot of great advice here, and there's no need to rehash it. However, here's an additional thought:
In the past, one of the factors in favor of a decision to turn pro put of high school was that, generally speaking, the coaching quality was higher in professional baseball than it was at the college level. This was frequently a selling point employed by professional scouts when talking with high school players and their parents.
I think that their assertion is still generally true; but, I also believe that the degree of difference between professional and the best college coaching is diminishing rapidly. This is particularly true at top college programs, the ones that tend to compete most often for draft-worthy high school grads.
A number of top college programs are making sizable investments in coaching expertise and technological resources that allow them to develop players at a pace that rivals the minor leagues. One sign of this is the greater ease with which college and pro coaches (especially pitching coaches) are moving between pro and top college ranks.
If this trend continues, the percentage of top players choosing college and the proportion of college players populating the draft ought to rise over time.