Don't Be Afraid to Cut Your Work.

This can apply in two ways

first, don't be afraid to leave work that doesn't measure up off of the demo reel, completely. You don't have to include everything, and if you have weak material that may drag the rest of your reel down, just cut it entirely. If you're just using it as space-filler to meet the minimum length requirements, odds are it'll be pretty obvious and your audience will lose interest quickly.

Second, don't be afraid to cut even your best work. In the course of your school career, you'll probably produce content that could be two or three times the length of your final reel; find the moments in the work that you choose to present that display your skills in the best light, and clip them. You can even cut more than one piece from an animation, and space the parts out depending on where they fit best with your music. There's no need to agonize over just how you're going to fit that dramatic, minute-long scene of a slow camera pan over the same landscape. Cut just enough for viewers to get a good impression of the quality of the work, and move on.