It is easier on everyone if you do just leave. A quick cheery "Bye, have fun!!" is always better than prolonging the separation. But, YOU need to be excited for her.
Tell her how you look forward to seeing what's in her backpack at the end of the day. Then, always end the goodbye with "have fun!" Because kindergarten IS fun.
She's anticipating the parts she doesn't like, and not the parts she does like. The more she dwells on her concerns, the more concerned she becomes. It's like going down the big slide at the water park. You know you want to, but you are kinda scared, and the longer you stand at the top and think about it, the more scared you become.
I know you are not doing this.... But, next year, on the first day of school, watch the kindergarteners. There are parents who Actually cry at the door until their child is crying too. Every year, outside of every kindergarten class, there are two moms standing outside crying. It's not funny this year, but, you'll see the slight amusement next year.