I like S.H.'s idea to find some books to read about going to school or preschool. Books are great for helping them understand and also forf starting coversation about his feelings. It's perfectly normal for him to cry at first because this is a transition and it's just becoming clear to him that this is going to be a regular thing.
From the teacher's perspective, here are some things that you can do to help. Have a routine that you do every morning when you drop him off. For instance, bring him in, put his stuff in his cubby, take him to circle time or his teacher 1 hug, 3 kisses, "I love you, have a great day, I will be back at (insert time here), bye". Then LEAVE. It's not that you aren't welcome in the classroom. You are or, at least, you always should be. If you aren't, run don't walk, to another center. It's that when you linger, the unspoken message is sent that you could possibly be talked into staying or taking him with you. Then when you actually leave your child will be even more distraught. I'm not, at all, saying YOU do this. I'm just saying whole process would be less stressful for everyone involved if everyone did. If you want to just sneak around the corner out of sight and listen in then do that. Actually, I recommend it. This will be very informative. You will hear just how long he cries, often it's only for a minute, or even seconds. You will also find out if the teacher is doing what she can to get him involved and happy. If she isn't, again, run don't walk to the next center. If a teacher isn't automatically available or the class isn't involved in a group activity, take him to a learning center and get him involved in an activity then one hug three kisses and so on. Talk the night before or in the morning while getting ready about what his favorite things to do are, or what he would like to do today when he gets to school then take him to that area of interest. Stay positive and upbeat because you don't want to indicate in anyway that you are worried or concerned. If you aren't sure this is a positive thing, he won't be either. Lastly, talk to the teacher about what she sees and find out what she feels would be helpful, that would make the transition successful for him and helpful for her. It always goes more smoothly when the parents and teachers tag team.
Hope this is helpful,
L.