Coaching HS is different than coaching TB. HS coaches have to play the hand they are dealt and cannot (legally) recruit players. Therefore, the coaches have to play more of a political game. I personally think that the best policy is to play the upperclassman unless the underclassman is "head and shoulders" better. Underclassman understand they need to wait their turn to start, while upperclassman may quit if their playing time diminishes. You see it in college all the time, an upperclassman transfers or quits if an underclassman takes their spot and there is no path to playing time barring injury.