There is always bias (even some Cardinals Manager sometimes makes head scratching playing time decisions
) but generally at the HS or college levels coaches will play the best players unless a player misconducted a lot. A coaches job security hinges on winning or losing and most HS coaches don't have that much talent that they can afford to sit one of the best players. Of course coaches have and ego and there are politics but in the end his job is to win and if he doesn't someone else will get the job.
Most of the time playing time decisions that are complicated will be about the middle of the back and fringy players, the best guys usually play all the time and often too much (pitching and catching the same game...)