I presently teach in a preschool (older 3's) and what you has said regarding tears, etc. is perfectly normal.  By normal, I don't mean that all kids cry at first, but it is not uncommon and most preschools should be willing to work with you on that.  I have a SW son who attended our preschool while I taught there so I have been down the road as informing re: meds, etc.  He has just entered kindergarten and has left me for the first time.  However, he did go on field trips without me so I had to prepare them for that.  Feel free to e-mail me privately if you have any questions.  My two kids have kept me pretty busy lately and I might not respond very quickly if you put it here.  I do try to keep up, but soccer season has run me ragged!  :-)