How Many Feet of Ladder Does it Take to Change a Lightbulb?

I don't know, I've never been good at estimation.
Note: See all the paper airplanes on the ground? Not my doing. Apparently the previous teacher let students throw paper airplanes up into the gigantic fan (see upper right in the pic) on my ceiling. I asked the maintenance guys to knock them down while they were up there changing the lights. Previous non-laddered attempts at dislodgement, including, but not limited to, both throwing chalkboard erasers at them and turning aforementioned fan on so high it shook and papers flew all over the place, had been unsuccessful. That table is also out of place. My classroom doesn't usually look so ghettofied.