To me it's pretty simple. In an 11-man school, you seldom see the offensive or the defensive coordinator, much less a positional coach leaving during the spring. The same can be said for most of the HC's in 6-man; and they all have one thing in common - They teach! In most 11-man schools, the AD/Head Football Coach is no longer in the classroom, so if they leave, the student's education is not at risk. In 6-man, and everyone else who isn't the AD/HC at an 11-man school, we need to stay in our classrooms to finish out state testing, which takes place in April/May. Most all of us know this, respect this, and if we're true educators, prefer this so we can give our students the best chance to pass as possible. But on a personal note; how do you leave your Seniors before graduation? I've never understood that philosophy. "Buy into my program, but I won't be there to support you on your biggest day."
Another way to look at it, is what are you implementing in the spring anyway? We don't have spring practice, and many of our students are wrapped up in track, baseball, tennis and/or golf until April/May. So really, the only things you would be implementing is off-season workouts and policies/procedures. Only so much you can fit on the chalk board, and mingle in during a 50 minute class, which also requires weights and conditioning.