No matter how careful we are when zipping up our jackets, we all have times where the edges get caught in the zipper and you have to spend minutes -- or Windex -- getting it out. With just a long piece of string or wire, you can prevent this from happening.
Instructables user Mr. H.S. Thompson points out that the "flaps" around the zipper are usually empty, meaning that something can fit inside, which then makes the flaps rigid enough so they don't get caught in the zipper. All you need to do is thread the string/wire through and you should be home free.
Unfortunately this doesn't work for most, if not all, pants zippers, so you'll still have to be careful and make sure your homework is put away before you pack up for the day.