Getting young players playing time is the most important thing toward developing them, how well they play is more of a secondary thing but if a player has 88 POT and they play a lot, and they consistently play well they may get up to 90/91 OVR.
If I remember correctly, players with a £6.5m Transfer Fee (I think £6.5 is the highest amount someone is valued at coming out of the Academy without playing a game) out of the academy usually will end up in the mid to upper 80's depending on their age when they come out of the academy, how much they play, how well they play, their position (if they're a CAM and you play them on the LW when it's not a secondary position they progress slightly slower) and what their OVR is when they come out of the academy, however the first and last details in that list are the most important in telling you how far a player will develop. Common sense dictates that it's going to be much harder for a player who is 18 and rated a 58 to reach his 90 potential than a player who is 18 and rated a 68 with the same 90 POT, the second player only has 30 points to gain, vs 40 for the first player.
I routinely call up players I know will never reach their potential for me because they're rated too low to get regular playing time anywhere and start their development but they still have nice transfer values due to their high potential (so, not all players, I release the duds, I just keep the ones with high potentials who I don't think will reach them), therefore I'll call them up just to sell them in the next transfer window for more money than they'll ever be worth to me (and will ever be worth at all in some cases).
This is a great blog that explains FIFA 12 scouting and academy procedures in depth and with a lot of detail, it tells you how to scout for the best young players, how to identify the best players in your academy, why certain players will or won't pan out and a lot of other stuff that's very helpful if you're going to be mining a lot of talent from your academy.
Also,
here's a website that gives you the potential of every player in the game, that particular list is for the higher level leagues, but if you click on the FIFA 12 drop down menu at the top left of the screen you can pick the "Lower Leagues" option, that'll give you a list of players who are young with reasonable transfer prices but still have good potential. You can organize it by column too, so you can have all the highest potential players at the top, or organize by country, league etc. Another very useful tool for CM mode that can helps you gauge how much a player is worth.