I feel like there are some common themes in this thread. First, you as a coach have to know where the power lies. Is your team big enough where you can dismiss athletes and still have a decent size team? Second, it is up to you as a coach to determine what is excused and what is unexcused. This should be communicated to your athletes early on. It is important to communicate your expectations to all athletes early on in the season. Whether the athletes listen and respect your policies is one thing, but at the very least you can say that you told them up front. It is important to be careful in the early years of building a program as you don't want to run kids off because you're too strict. However, once you get the program up and running and you've got a decent size team, then it seems like strictly enforcing your policies becomes easier. Also, as noted all of this becomes easier once you've started winning. People want to be apart of a winning tradition/culture, and will do much more to make practice when this has been established.