Reddit mentions of Reel Success: Creating Demo Reels and Animation Portfolios

I'm gonna sound like a broken record here, but it really does depend on what you personally want to end up doing in animation. Your portfolio can be flavored in many different ways - someone who puts a lot of animal/creature animation in their reel will probably end up doing a lot of that, combat/action will end up doing it professionally, etc. If you're looking for a generalized portfolio, I'd recommend this book. I think overall what matters when trying to get into schools would be showing a variety, and that your pieces look good and finished, even if there's only a couple.

Additionally, every school is different. Some animation schools don't even require a reel or anything to get in. If they don't, research them carefully and make sure you know what you're getting into. And don't worry too much about your reel being 'perfect' - that you have anything at all prior to entering school already gives you a leg up over other people.

Finally, once you're there, absorb everything you can. Take advantage of every resource available to you, and connect with people from all different majors and mediums. You never know who you can help and who can help you in the long run. I was often told when I was in school "It's not what you know, but who you know." Establish a good working relationship with people and you probably will always have a helping hand in the industry. Good luck!