I doubt the students would actually learn humanities in the extra time allotted if it's not used for math. I remember a distinct refusal to internalize, especially in my male peers, during "English" classes.
Forget humanities. The hours a week after school that highschool students spend on calculus homework would probably be better spent socializing with their friends. They'll never be young again, wasting the time of a teenager with unproductive busywork is a horrible thing to do.