RE: Why would school say no rolling backpacks?
I used to teach elementary school, and we had a no-rolling backpack rule also. One reason is that most kids would just pull them behind them without being careful and so others would often trip on them duing the very busy/crowded morning and afternoon hallways. Plus they seemed to always flip over while the kids were pulling them along. I have taught both first grade and kindergarten and we only sent home a single folder each night, so the max a child would have in their backpack would be the folder with some papers in it, maybe a second folder one night a week with graded work/newsletters, and a library book- not too much to have on their backs at all. I wouldn't worry about it until it becomes an issue for your child.
M, mommy to A 2005, E 2007, and L 2010