Hi,
I'm a nUb in the biking world but having graduate from college last month I can confidently say that carring a backpack with hardcover science textbooks, binders, water bottle, etc, will easily add 10 pounds or more. Lighter traveling is possible though. As others have said, I doubt the bike will break unless you're jumping off the stairs or something.
Anyways, have fun losing weight! College is def the place to do it. Most uni's have exercise facilities on campus and have lots of opportunities to be active. Sadly, most people gain weight after graduating and finding a job because they become more sedentary.
later, jUng