Right. There's some words about it in the Tactics part of the Game Manual.
(http://www.buzzerbeater.com/community/rules.aspx?nav=Tact...)If you don't want backups or reserves who are better to play, then don't do "Let Coach Decide" and go with "Strictly Depth Chart". That way they'll never jump the guys ahead of them (not counting stamina or foul trouble).
The easiest way to get your trainees the 48 minutes they need is to play your scrimmage and have your trainee in all 3 slots (starter, backup, reserve) so that no one will be subbed in for him - unless he fouls out of course, which is rare.