Work Expands to Fill the Available Space

I usually spend about 10 hours inside the school building on any given day.  That means 50 hours/week.  Add in the time I spend on the weekend and you're probably up to about 55.  Which sounds like a more-than-adequate work week.  Yet I can't figure out how that never seems like enough time.

And trust me, those are ten badass hours.  I usually work through my lunch, or if I do stop, I only take about half the period.  Factor in a bathroom or coffee break here and there and I'm still working for nine or nine-and-a-half solid hours.  How is that not enough time?

I don't know, but since I'm not Jewish, I suppose I'm doing my own personal version of the Day of Atonement in my home office today with a nice thick folder of student essays and two lesson plans to write.  And even so, all that work, which will likely take a few hours, will only get me through Friday.  They say that work will expand to fill the available space, and I think that's true, but I'm not doing much beyond what I need to do to keep my head above water, either.  I don't advise a club or serve on any committees; since I'm teaching a brand-new class this year, I decided I need to have time to focus on planning.  And it's turning out to be pretty demanding.

I realize that this is hardly breaking news to any of my colleagues, but as always, I actively seek tips on time management.  I've clearly mastered the "saying no" part, but I'm wondering how I still don't seem to have enough time to do more than just barely stay ahead of the curve.