I work at a day care and we see this ALOT! Most likely she cries when you leave, and during transition times, like coming in off the playground, or going in to nap time.
What works well is to make a family photo album. Just a small one you get at the craft store will work fine, put in pictures of EVERYONE in your family, especially ones of her at happy times with you and dad and let her take it to school each day. Being able to get it out and look at it when she is sad helps.
Also if she has a special blanket at home, cutting a small square of it off and putting it her pocket might help alleviate some of that stress.
Lastly try the book, The Kissing Hand, it's about a raccoon who is scared to go to school and how mama makes it better for him by kissing his hand so he can feel her love with him all the time. My son's Kindergarten teacher read this the first day and had ALL the parents do it as they left. It really helped him adjust. Making a goodbye routine with kisses, hugs, and a special something like the kissing hand will help her through the day.