50 lb + 15-20 lb has to be an incorrect estimate. The average person can't hump a 65-70 bp backpack. From my backpacking days, that is a full size backpack loaded with food, water and gear for a week.
No way the average student is carrying that load. OP doesn't need to and shouldn't carry his entire library to school every day.
Anyway, a standard bike with 65-70 lb of weight on the rear will handle like a live pig, doesn't matter what kind of tires, rack, etc.