I have been a preschool teacher for almost 8 years. My tip for young children going back or starting school is a "ME" book. Prepare with your child a book all about themselves. Add a pictures of them when they were young, pictures of Mom, Dad and siblings, pictures of other friends and family, pictures of summer, and other pictures that the child might choose. On a the final page add a message to them of encouragement. This book needs to be small. Half of a sheet of regular copy paper is what I recommend. First this book is a way that they can share their family with their class. Then they can keep there book close by to look at when they miss their parents or have an emotional moment.
As teachers we can prepare a Friends and Family board. Ask parents send small picks of the child's family and friends. You have the child make a mini poster. These foster a sense of belonging in the classroom and can help when they are having a bad daa "ME" booky. You can snap a picture of the child and their parent on orientation day. I have also print pictures from a parents Facebook page. (Ask permission