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The German word for book bag is "Buechertasche".
la mochila is how you say book bag in spanish
Mark Bridges has written:
'The Gig Bag Book Of Arpeggios For All Guitarists (Gig Bag Book of)'
'The Gig Bag Book Of Guitar Complete (Gig Bag Books)'
Bolsa de libros is "book bag"; you can also put your books in a mochila, a knapsack/backpack.
A backpack is exactly the same thing. A book bag is not just for books you know.