What my friend carries to the school

Learning or knowing synonyms is never useless. "School bag" is not a synonym of "backpack". A backpack is a bag that you wear on your back. A school bag is a bag that you take (took, had taken, used to take, etc.) to school. Your school bag may happen to be a backpack and your backpack may happen to be a school bag but not necessarily.

knapsack, used to be just a rolled up blanket slung over your shoulder, with maybe a few food items rolled up in the middle.
Take a look a some pictures of Civil War soldiers, if you need a look.
Very practical. Nowadays, it generally means a bag of sturdy material and furnished with shoulder straps, designed for carrying articles for camping, such as a blanket or sleeping bag, on the back

Haversack, is a bag for oats or oatmeal. But because Oats or Porridge made from oats used to be a common travellers fare, it also came to mean a bag or case, usually of stout cloth, in which a soldier carries his rations when on a march.
It is distinguished from a knapsack, in that is smaller and not necessarily with the straps needed to carry it on your back.

Which civil war?