I had to stay extremely organized because I've had to pack up my entire classroom every year for the past 7 years of teaching (bad economy = lay offs based on seniority, grade level changes, carpet removal and remodeling of school, etc.). My 8th year (this year) will be the first time I won't have to pack everything up! Phew!!! And the year I had a baby too! It couldn't have come at a better time. :)
Anyway, I'm sure if you've taught a few years, or even one, you know some things you will want to continue buying for your classroom year after year. To ensure it's ready at the beginning of the year, these things need to be bought during the summer. With so many expenses, you never want to buy too much, just enough for your anticipated class numbers, plus some. We teachers are already semi hoarders. Let't not add to it.
I basically have a chart that I fill out in June. Then when I've had some wine and down time, I start thinking about school and prep by buying needed items. To avoid having to go to campus just to see what I have and don't have (In my case in years past, everything was in boxes, so it was near impossible.), I fill this out in June before putting everything away. The count for what's leftover in June can also be approximate or if I KNOW I have enough, then I don't even bother counting. This just helps me purchase what I need and not have a ton of extra leftovers. Also, I tend to save on shipping costs if I purchase most things together.
You DON'T want to have to dig through everything just to figure out what you have/need. Yup, this is what my dining area looked like for a few years in a row.