Is this immodest? My opinion is that it is not.

By your own analysis, there is no reason for the skirt to be so short, she is not playing any sport that requires a short dress.
By my analysis, there is not much to see in the picture, it is not clear. The skirt could be tight and thin and she is wearing a G-string that is clearly marked. Maybe the stocking only go up a little past the skirt and when she sits you can see the upper thighs, like French maid outfits.