Not sure who to address here...
Pick her up. Sympathize with her. Don't try to stop her crying. She is sad. She has left a person she has spent a lot of time with and will not see her again. She is grieving, and she needs to spend the time being talked to like she's lost someone, even though they aren't dead.
Most children see their teachers as living at school.
Try talking with her about what the teacher is doing now and where she is going, and then talk with her about HER summer and what she is going to be doing - even if it isn't 'much' in your eyes.
You can't replace the teacher. You can only be there for your little girl who has just suffered a loss.
Most people expect kids to get over this stuff. After all I have learned, that's just BUNK. Your daughter's tears are just further proof. : (
Good luck, and hope you find some common topics to transfer to after talking about 'things' the way they are now.
M.