These phases are really normal. Acknowledge her feelings and empathize; don't shame her for feeling sad. Big girls DO cry. Hell, I cry sometimes and I'm almost 40 years old. Crying at dropoff is not misbehavior, it's how a little kid expresses that they love you and miss you, and that transitions are a little hard right now, etc. If you are empathetic but calm, you will both make them feel heard and reassure them.

My kid goes through this every once in a while, and I usually agree with her that it would be really fun if I could stay in her classroom all day or she could come with me to work, then remind her that I have to go to work, that she will have fun with her friends, and that I will pick her up in the afternoon. And I give her a big hug and, if necessary, help her get set up/join in some activity.

There are also books like The Kissing Hand that you can read and talk about.

Mine thought I would die and never come back...every day - so sad for about ten mins. They get over it fast. In cried for about 20

Dear OP- my DD did this thru K and it was grueling but necessary so buckle up and enlist the teachers at drop off to engross your DC in an activity when you leave