Control Your Emotions, Coach
Young athletes are not adults and do not have the life experience to be held to the expectation of being able to control their emotions. Sports gives a student a controlled environment to learn how to manage feelings and emotions, and the coach is the teacher. That teaching is one of the biggest lessons a coach can teach an athlete under their supervision.
Increase Emotional Intelligence with Intentional Interruptions
Start on time, every day. End on time, every day. Teach the athletes what to do if they are late. Don’t make them run when they are late, that just makes them more late. Assign a team captain to facilitate consequences after practice for those who are late. If no one is late, the captain doesn't have to stay. It only takes once for a captain to have to stay after because of his or her teammates. Peer pressure is WAY more powerful than whatever you have to say about the subject.