Since this topic comes up occasionally, I thought I should quantify it somewhat to see if the data supports the conclusion that I reached quite some time ago that the one and done rule the NBA imposed was arbitrary and dumb.
The list of HS players drafted into the NBA isn't very long, so data-driven conclusions are unlikely to reach very far unfortunately. But they are still useful to examine to see if there are any hints. I would suggest that we are likely being imprisoned by media narratives surrounding parts of the limited data rather than the whole from this analysis.
Starting just with the 1995-6 classes, we can see right away two Hall of Famers, both clearly the class of their draft class. Kevin Garnett is leaps and bounds above any other player drafted in 1995. The next best player in that draft is Rasheed Wallace (drafted right ahead of him at 4th), but he's way behind in value.
The following year we get Kobe Bryant picked 13th, again the best player of that class, but it's a much better draft. He's followed by Steve Nash, Ray Allen, Ben Wallace (who went undrafted), and Marcus Camby. Iverson and Stojakovic are also better than Rasheed was, for comparison of how deep this class is. Jermaine O'Neal was also in this draft at 17th and falls in a class behind these players but still had a respectable career (if injury riddled and complete with an infamous fight). He is roughly as productive and valuable as Stephon Marbury was (a one-done shoot first point guard who went 4th).
1997 Tracy McGrady, picked 9th, is the 3rd best player in a draft following Tim Duncan (just ahead of KG's class as a top tier Hall of Famer), and Chauncey Billups (a pretty clear HoF candidate too). It drops way off after those three (Antonio Daniels?).
1998 Finally a bust? Al Harrington as a HS pick is rather mediocre, but he was picked 25th. And despite being a rather average to below average player is still playing (because he can shoot reasonably well). Rashard Lewis picked 32nd is a capable NBA player and 5th best player in this draft (roughly as good as Antawn Jamison who went 4th). Dirk Nowitzki is an international player of note picked at 10th is the best player in this draft, followed by Paul Pierce and Vince Carter. Brad Miller goes undrafted and comes in at 6th. Korleone Young is drafted 40th and does pretty much nothing. Second round picks rarely do though. Famously this is the Olowakandi draft (he was terrible and picked 1st) and also famously Nowitzki was traded for Robert "Tractor" Traylor, who went 6th and has a short unimpressive career while Dirk has a surefire HoF career.
1999. Here some real worry probably started to creep in. Jonathan Bender goes 5th, mostly on the strength of comparisons to KG. While he never really puts up any numbers anyway, he also has a very brief injury riddled career. Leon Smith is a total basket case of a human being, is drafted at 29th and doesn't amount to much either. More than likely, his career, what there was of it, IS the case against drafting HS players. But I have a hard time seeing much generalisation to be made of his circumstances to any high school athlete. Manu Ginobili goes 57th and is probably the 2nd best player in this draft (behind Shawn Marion).
2000 A historically bad draft class all around. Michael Redd is the only good player drafted (43rd), while Mike Miller (5th) and Kenyon Martin (1st) are the only other reasonably productive players picked and a pair of others (Turkoglu and Jamal Crawford) are decent. And that is all. Darius Miles goes 3rd, the highest a HS player has gone so far, probably because nobody else seems very good anyway, and doesn't amount to very much (but isn't nearly as bad as Marcus Fizer, picked after him at 4th). DeShawn Stevenson goes 23rd, and appears to have a niche as a bearded defensive player (and is still in the league today) but few other appreciable skills.
2001 Odd class all around. Kwame Brown goes 1st. He is a below average player, but sticks around in the league still today and isn't nearly as terrible as Olowakandi was. Pau Gasol is clearly better than anyone else from this class (picked 3rd), followed by Tony Parker (a young French point guard at the time, picked 28th), and Tyson Chandler (picked 2nd and another very good HS player). Despite the flops of Brown and Eddy Curry (a HS player picked 4th, a high volume scoring big man who can't do anything else at all, ends up with a heart condition to boot), this is actually a very deep draft with many highly skilled players scattered in it (about 11-12). Diop, while styled as an international player, played HS ball in the US and is picked 8th. Doesn't amount to much.
2002 Amare Stoudemire becomes the first HS player to win a rookie of the Year award after being picked 9th. He remains the best player from this draft, although Yao Ming was too injury prone to actually claim this title. Stoudemire's a problem child player for much of his career though and a terrible post defender.
2003 LeBron. Enough said there. Kendrick Perkins goes 27th and isn't all that productive but has a mean scowl and excellent post defense skills. Travis Outlaw goes 23rd and somehow ends up with a "Travis Outlaw's contract" style career, which is to say, he gets dramatically overpaid and isn't a very good player. Darko Milicic goes 2nd and isn't all that productive either. We could consider him either international or HS. Ndubi Edi and James Lang are also HS players drafted in this class. Neither amounts to much of anything. Both go late (26th and 48th).
2004 Dwight Howard. Again, enough said. A ton of HS players are picked this year in the first round. Al Jefferson goes 15th and is the 3rd best player in this class (behind Iguodala). Josh Smith goes 17th and while something of a coaching problem (for putting up ill-advised jump shots, among other reasons), is actually quite good (probably 6th best in class behind Luol Deng). JR Smith is an even bigger coaching problem at 18th, but again, at least a skilled scorer. Dorell Wright goes 19th and takes a while to get some PT, but turns out okay (better than JR, but with less minutes). This class is marred basically by the Bassy Telfair pick at 13th, who had a documentary film produced about his HS to NBA leap and the knee injuries to Shaun Livingston (picked 4th). (don't click on that if you're squeamish, his leg literally bends sideways suffices to say). And then the pick of Robert Swift at 12th, which amounts to a total dud, but certainly not any bigger than Araujo at 8th.
2005 Andrew Bynum goes 10th, aside from injuries, he's probably the 4th best player in this class (just ahead of undrafted Jose Calderon) but well behind Deron Williams and way behind Chris Paul. Monte Ellis goes 40th and becomes a famous ballhog but otherwise is a solid player (especially for a second round pick). Louis Williams goes 45th and ends up as a very good instant offense shooter as well. Martell Webster goes 6th and ends up just below average (behind Francisco Garcia who went 23rd). Andrey Blatche is a headcase picked at 49th but having a productive season this year with the Nets. Amir Johnson goes 56th, gets limited minutes with the Pistons, but plays very productively in them (followed also with somewhat less limited minutes with Raptors and still more productive play). Gerald Green goes 18th and wins a dunk contest, but not much else is done. CJ Miles goes 34th and is also largely unproductive.
I don't count very many "busts" here such that the problem was NBA teams needed to protect themselves from themselves. There's a good volume of late-first round and second round picks (picks which rarely turn out to be suggestive of productive NBA careers). This suggests that many NBA teams already recognized the dangers of HS evaluations and adjusted accordingly. The famous busts of Kwame Brown (thanks be to Michael Jordan), and Telfair's documentary, and a few problems of immaturity (Miles, Stoudemire, the Smiths, Blatche), appear to be the cause of concerns here. There are also numerous cases where these players had to play a year or two of pro ball to begin to become productive however. Garnett or Kobe don't start out as a perennial All-Star for example and McGrady doesn't win a couple scoring titles right away either. Howard or LeBron are both very good immediately but neither is rookie of the year, and neither is in the discussion for top 5 best players in the league for at least a couple of seasons. So it is possible an extra year of seasoning may help these players mature better or polish their games. That argument is not invalidated by saying the age restriction is dumb. It also takes most college players a year or two to begin to adjust to the skill level jumps, and usually year 3 is when any player begins to really flourish.
What it suggests is the NBA prefers to have someone else pay to develop the players, pushing risks off on someone else (in this case, the players themselves and colleges who recruit them for one-and-done runs). For the most part, college freshmen who are drafted are a relatively safer bet than HS players, in the defence of this practice. But college freshmen are rarely drafted outside of the lottery or first rounds. HS players were. In addition, statistically valid issues can be raised with international player scouting and that of 4 year seniors who are drafted, both groups that fail on a very high rate relative to HS players and underclassmen who are drafted. There are prominent examples of both groups who succeed, who are held up as exemplars (particularly Tim Duncan or Steve Nash types as 4th year college players) but we rarely hear about 4th year seniors who are terrible professionals who are drafted. This may be because such players are rarely lottery picks in the first place (Jimmer Fredette so far is in this category, Terrence Williams from a few years back, Acie Law and Al Thornton in the same draft, and then Luke Jackson, as but a few examples). But if such a comparison is fair and honest, we should admit that many of the HS players who have failed and busted or been mediocre are of similar draft status to that of international or college players and have similar expectations leveled upon them. Whatever the valid reasons may be behind such a policy, it doesn't appear that "protecting" owners from their supposed stupidity is one of them.
HS great picks would be
Lewis (mostly because he was second round)
HS Busts would be
Brown (#1 picks aren't supposed to be below average)
Lost picks would be
This looks like a fairly normal distribution, but it has some serious top heavy options.
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