The word for school bag is "sac à dos" (pronounced "sak ah doh"). "Sac" is masculine, so "sac à dos" is also masculine.
Here are some examples:
* J'ai un beau sac à dos. (I have a beautiful school bag.)
* Il faut acheter un nouveau sac à dos pour les cours. (We need to buy a new school bag for classes.)
* Mon sac à dos est trop lourd. (My school bag is too heavy.)