L.C.
My daughter has been a preschool teacher for a couple of years now, and she uses the sticker reward system for times like this.
Take some stickers to the preschool teacher and have her reward her with a sticker (privately) when she was brave enough to use the potty there. Yeah it's bribery, but it works most of the time.
You might want to see if the potty's there are like the one at home, or are they the more industrial type with a stronger flush. She may be intimidated by them if it's the industrial type. If so ,she may need more coaxing than the teacher is giving.
I'd visit the preschool director and discuss this with her. She may have some ideas on how to overcome this fear.