If your preschool is a good one, I am sure they will be helpful.  It is hard to see tears, I know. I try to have things waiting at the table for my kids to do so that they look forward to that first activity.  Perhaps if you know she loves playdough or puzzles, that would be out and ready for her and you can give a quick kiss goodbye and assure you will see her soon.  You would be surprised how quickly most kid stop crying.  And the longer the parent lingers, the harder it is for the child.  Lots of bargaining, etc.  It just drags it out.  There are so many things children gain from each other in a social setting.  Our preschool is one that encourages learning through play and are willing to provide so many of the messy things kids love that we , as Moms, don't want to do in our homes.  When she gets over that hump, she will benefit from the experience.  The public school has a medical form that our endo filled out to give oral Cortef in certain situations.  Perhaps you could work together to come to a common ground in that area so that they can give oral Cortef until you arrive.

Good luck!