la mochila is how you say book bag in spanish
To say 'bag' in Spanish, you will say 'mochila'. This only applies for school bag. The general term for "bag" would be "bolsa".
Bolsa de libros is "book bag"; you can also put your books in a mochila, a knapsack/backpack.
saco
Bolsa de suciedad
Libro de texto(Literally "book of text")
you say it `este libro'.
libro
¿Qué hay en tu cartera
"on the book" "en el Libro"
El Libro= The book Libros= Books
cuaderno
nuestro libro