This is a great topic and a critical one for success in any endeavor. What we have seen at 3P Sports is this is a problem for little leaguers right up to the pros. We are working with several pro pitchers and we often discuss the need to focus on things like "one pitch at a time". Too often they drift mentally and think about the hitter on deck or the bad call an umpire just made, etc. The universal guidance we give is to use the proper self talk... Like saying to themselves... "one pitch at a time...I am going to execute this pitch down and away".
Teens will struggle with focus...because they are teens! Encouraging them to execute positive self talk will help. Positive self talk keeps their attention on the task at hand.