My daughter has had similar issues which I ended up addressing with the school adjustment counselor. She had mentioned that some children don't know quite how to put there finger on it or articulate what the problem is, but are sometimes very fearful that something might "happen" to their mom or dad once they leave them at school. It can sometimes turn into a preoccupation with death or the parent just not coming back. Do you think she could be worried about you? Has she maybe seen something of this nature on t.v.? Anyway, just something to think about.
I ended up seeing a difference in my daughter after she changed school and the morning routine was somewhat altered. Now she lines up outside with her whole class and then enters the building, rather than going in on her own. I've found this has made a huge difference when she can enter the classroom with a friend or two.
Another thing I had tried that might be helpful was a behavior chart. I posted if on the refrigerator and explained to her that she would get a sticker for every day she was able to go to school without tears. When she received five stickers, I would then take her to the Dollar Store so that she could pick out a prize. This seemed to help as well. The reward doesn't have to be something purchased, but just something that she gets excited and motivated about. Once she starts earning stickers consistently, you can then up the number of stickers she must earn before getting the reward.
Anyway, good luck. Hopefully this phase will pass quickly!