In my opinion, you're on the clock for your clock time, and for any scheduled out-of-school commitments, such as contests, concerts, or marching activities.

At EOD Friday, I don't check my email again until Monday morning. I'm not available on the weekend, full stop, unless previously arranged. Literally nothing will happen on Saturday at 2PM that I can do ANYTHING about until Monday morning ANYWAY, so why check email or even worry about it?

Your HBD may be a workaholic, but that doesn't mean you have to be.

ALL that being said - find a new job. Your HBD and you have incompatible work ideals, and you won't ever agree.