Dear Person Who Asked Me to Volunteer at School,
The problem is if I say yes to you now when I’m feeling charitable and have a light schedule and the sun is streaming in the window that does not guarantee that on the date 12 days from now when you need to count on me being chipper and active and fresh that I won’t be miserable running late juggling a deadline and resentful of all the other mothers who have no job and plenty of time to wash their hair and volunteer so I am saying no.
verysincerely 
This is such a problem for writers! I once heard a talk by a stay-at-home published author mother who decided that her writing IS her volunteer work, in that one is rarely fully compensated for it, and that this is her true offering to the world. I’m sticking with that!
Oh, I love this! Writing IS volunteer work!