Perhaps our very favorite, is from Anna Dewdney's Llama Llama series, Llama Llama Misses Mama. It's Llama Llama's first day of preschool and Llama Llama couldn't be happier, that is, until Llama Llama realizes that Llama Mama isn't staying. This is the story of what I imagine will happen on our first day of preschool, in hardcover. Funny, sweet and, ultimately, reassuring for both Little Llamas and Llama Mamas, this is a must-read for any preschool bound-kid and her Mama.
Perhaps one of the sweetest explorations of dealing with the differences that seem to get highlighted in a school setting is Yoko by Rosemary Wells. Yoko's just like the other kids--until lunchtime, when her sushi causes quite a stir. It's a lovely lesson in appreciating differences.