This really depends on the grade/age you're schooling, but we seem to always run out of glue sticks, printer paper, toner, and dry erase markers about 3/4 of the way through the school year. I bought extras this year. The colored pencils have a tendency to walk off, too, so I may have to get more of those. Pilot G-2 pens in bright colors are my favorite pens, so when I find a sale on them I tend to get a pack or two. I'm stocked up from a sale mid-year last year. Same with mechanical pencils, but I buy them periodically through the school year because they do tend to go on walkabouts at the most inopportune times.
My dh is at he end of the hall in his department and when a visiting prof cleans out an office, the supplies tend to get deposited in his space. Which he dutifully calls me about to see if I need anything.
I haven't bought binders in 3 years, even though we use them a lot! I love those see-through colored plastic folders that he got me stocked up on 4 years ago.
Come to think about it, I probably should buy more binder pockets. I think Littlest could use more in her Kindergarten binder.