If your using a bag for school then you have to get the large size with the long handles, it's $132.00. That's the one that's going to fit everything inside. The medium one is too small, it's more of like an everyday bag to through your stuff inside.
I have a medium with short handles and had a large with long handles [hated the color]. Long handles can be held, short cannot be on the shoulder. Medium holds a lot but not so many books, works as a toiletry bag though.
Leather can be washed. I have washed stinky old Coaches, a couple bent out of shape Dooneys and an ancient Ghurka that was even lined. Don't know if I am brave enough to put in the washing machine but hand washing is fine for many leathers.