Always remember that graphic design is a service-based business. In most cases, your client hired you because they are stressed out and need your help; giving them "homework" assignments only adds to their stress, and makes you look incompetent and unprepared to handle every aspect of their project.
Of course, there will be times when you need to ask your client to do things. In these cases, you can try to make it as easy on them as possible by gathering or prefilling as much of the required information as you can.
- From Burn Your Portfolio by Michael Janda
The goals of teachers and coaches are centered around academic and athletic learning, growth, and performance. The job is a little different from the graphic designer's to be sure, and sometimes, for coaches and teachers, assigning a little 'homework' is unavoidable. For a teacher, this might look like assigned readings, research, reports, and practice drills. For a coach, this might take the form of restorative work, viewing game performances of themselves or others, and practice drills.
Yes, 'homework' is a fact of life for students and athletes, but it can EASILY (and very quickly) be overdone. There are times when homework is NOT effective, or even appropriate. Assign homework if you must, but make it thoughtful and use the tool sparingly.