I have been preaching this for many years.
I used to be that director that spent all of his time at the band hall. I felt the more I did, the better my band and my students would be.
Now I teach what can be done in class. I leave when the bell rings at the end of the day. If a student wants to come in after school to work, I will stay and help, if not, I go home.
Too many teachers work too many hours for free.