Twins at preschool-not settling in?

My twins have just started preschool and their teacher says it is becoming problematic as one twin has taken off with a new friend and is being nasty to her sister. My girls are the best of mates at home but it seems in school the more dominant twin is settling in well whereas the quieter one is unhappy and tearful as she's feeling pushed out. My own opinion seems not to make a big issue out of it, whereas the teacher keeps bringing it to my attention everyday. Anyone been in a similar position? Any advice would be great.