She'll have to change from some sort of pants into the skirt/dress at school (or hide them beneath the skirt) if her bike does not have a skirt-guard on it (almost all modern bikes do not!). The skirt could get caught in the rear wheel otherwise, especially if it is long and/or looser-fitting. You wouldn't like the result...nor would she!
Does it have a chainguard/chaincase? If not, many of the same arguments apply as to the skirt-guard for her, and add in tears and/or grease damage from the moving parts (chain and crankset) if she should even rub against them (easy to do with anything...even skin...lol!).
Never you mind mounting the bike if the frame is not some sort of mixte/step-through frame style!
Modesty...while so important for anyone, not just women...is really the least of it here.