First of all I would set the backup and reserve to be the same as the starter. Second of all its easier to train 5 players than it is to train 6. I train 6 and occasionally (only happened to me once due to a foul out) something will happen that causes a player to get subbed in. Most often this can be fouls or injuries, but the in game coach can decide that you are winning by a lot and so sub in your other players. There is no way to control the in game coach. Just set Let them Play, Set Strictly Follow Depth Chart, Set the Backup, Starter and Reserve to be the same person (the trainee) and hope for the best!
Hope that helps!