Those suggestions that have been made here are very good.
The very first thing that I would suggest is to sit down with every player, ask them for a nutrition itinerary, that they had that day.
what did you have for breakfast this morning?
what did you snack on b-4 lunch?
what did you have for lunch?
what did you eat prior to coming to this session?
Normally, during the preseason a player will end his last class, hit the lockers, change, then show up on the gym floor, or, onto the field somewhere.
Take special note of those that just seem to be running on a few cyclinders, sluggish, off the mark. If you have any experience in this area, you eyes will pick out those that either don't make a deliberate effort to prepare properly, or even worse, those that lie. It's the group in the second case that deserve a = " how come?"
It's not all that unusual for a youngster to feel like there's no big deal missing a meal, grab a quick fix of a candy bar or something. They deserve better and you can set an example that very few high school programs provide - VERY FEW.
Carbs are needed prior to any workout, proper hydration also - prior to, during and after. Other nutritional needs can be addressed.
Want some suggestions - ask away.