I can definitely see situations in which it would be necessary for a high school coach to be calling pitches, but in general, I disagree with it. Being a junior in high school I've been on the mound now for a good 7-8 years (I can't remember my first days of pitching). By this point in my career I know and understand my own tendencies and also know my own comfort zones. I know that if I get the other team's #4 hitter down 0-2 that hes going to have the discipline not to swing at a weak changeup off the outside corner, so why waste the pitch? Coaches get too hung up in "chase" and "waste" pitches that they end up digging their pitcher a hole. My motto has always been that if he couldn't catch up to the fastball on the first two pitches, who says he's gonna be able to on the third?
One of my biggest peeves about coaches calling pitches is the amount of pressure that it puts on the kids on the mound. Yes, we've thrown the pitch hundreds, maybe even thousands of time in practices and bullpen sessions, but many kids have the tendency to crack under pressure. For many guys, even myself at times, it's a real challenge to throw a quality change over the outside corner no matter what the count, batter, even if they're in the game or not. The challenge alone is enough, then add the pressure of a coach screaming at you from a dugout to "hit your spots!".
Plus, espescially for a younger guy, it helps to know how you react in certain situations, but when a coach is making all the decisions on the mound, how can you learn? Yes, I see the practicallity in coaches calling pitches for kids ages 10-13 maybe 14, but once they start getting older they should know by then what to do in different situations. And if they don't it isn't a reason to yell at them from the dugout, or bench them, it should be seen as an opportunity for the pitcher to learn and grow. I believe that many times coaches subsitute discipline and punishment in the place of learning and growing.
Enough of my ranting! Haha! Thanks to everyone that posted before me, you all have excellent opinions that I took into thought as I was typing this.