For me, the three books that every school library should have are:
1) The Giving Tree, because it is a classic Shel Silverstein book that teaches you that love can make you give someone everything you have.
2) Oh, the Places You'll Go, because it is a classic Dr. Seuss book that teaches you not to be afraid of change, even though it can be scary and full of bumps. Change is good!
3) Where the Wild Things Are, because it is a classic Maurice Sendak book that teaches you that it's okay to be yourself and it's fun to be wild sometimes!