Great post, Coach, and it's good to read somebody talking about high school baseball!
I hope you don't mind if I add a few that would get my attention, if I was a HS coach.
1 - be the first one out of the dugout going after a foul ball. Coach should not have to say "somebody on that" more than once per season.
2 - After an away game, make sure you're carrying a bucket or a bag to the coach's vehicle.
3 - Keep your grades up. Don't make the coach wonder if you'll be eligible next quarter.
4 - If you're a bench player, anticipate the game situation, and be ready. Don't be scrambling for your helmet when coach puts you in to pinch run. If the catcher is on base, be ready to grab a catcher's mitt and mask.
5 - 100% focus during games. If the coach turns to you to confirm what the count is, and you don't know, you've let him down. If you're batting with an 0-2 count and you don't know what the pitcher's been doing in that situation so far, you've let yourself and your team down.
6 - Know the signs. Call time if you're unsure. There's no excuse for a HS player to miss a sign. But it happens all the time.
7 - Be a student of the game. Know the situation and what it means. There's no excuse for a HS player not to know what base he's throwing to or how many outs there are. But it happens.