I have both the le pliage and the planetes in the large 'shopping' size and to me there isn't any difference between the two in terms of size. However, the material and foldability differs. The Planetes can't be folded like how you would a le pliage but I noticed that the material of the Planetes is thicker and seems sturdier than the le pliage. The Planetes is also tone on tone leather meaning that the leather strap and flap would be the same colour of the fabric as opposed to the regular le pliage which has the brown leather flap and strap.
I think the large size w/ long handle is perfect for school! It really would fit everything! You can go for either because to me they seem to be the same (even the handles and flap) it's really down to what colours you like etc.