The very first time you meet the teacher, after the (!) mail comes, hit A when the teacher comes on the screen. The teacher will walk away and a new teacher will come. Keep going until you get the teacher you want. Once you've had this mail, the teacher cannot be changed.
You cannot choose your charater for pre-school, which i assume you are talking about because mrs. flower is a pre-school teacher. But her name is not that. Mrs. flower is school teacher and is not fat. You can only choose your school teacher. When you tama gets the mail saying it can go to school press A to change teachers.