I tried doing that when I started out as well but its not viable. If you're not training your young players you are losing out on their transfer value. If you want to maximize on player's potential abilities and transfer prices this game offers no option other than to train either guards or big men as groups over an extended period of time. In the meantime for whichever group you're not training, you should sell your young players and replace them with older experienced ones. Maybe doesn't make a lot of basketball sense but its the best way to do well quickly in this game.
On the other hand if you are determined to train all your young players team training might be an option, but one that will lead to a lot of useless training - for example, shot blocking for guards.