Hey N.,
I am a preschool teacher. One of the best things to do is find a good center that doesn't exclude children because they aren't trained yet. I have helped train many children over the years while they were in my class. Both my children had wonderful teachers who worked with me on this issue as well. That's the tricky part: getting a teacher who will work with you to develop a toilet training regimen. For my son, he would sit on the potty, but found an older boy to model standing up to go. Another mother came to me to say her son was now using my son as his role model for standing! My daughter trained easier, but she has the disposition to wet herself when she was mad at me. Her teacher, (who I affectionately call Jay's other mother) was patient with her, and it helped a lot!
At home, I used sticker charts and special events as incentives. Keep trying, and don't push too hard. Your child will eventually "get it".
S.