i need some explanation here: are they sitting on the wall or why do you translate this sentence with πάνω? I would translate: ...στους τοίχους
It does not matter where the girls are sitting, but where the action (write) is done.
It is also right to translate without the local adverb "πάνω/επάνω" (on the surface of a thing). Instead of "πάνω στους" we can only say "στους" (its general intention "σε" which denotes place (stand or direction))
The same thing happens with "μέσα στο (in to)" that we can simply say "στο". That is, the words "μέσα" and "πάνω" can be omitted if you mean by context.