Once you've selected (or been appointed) a teacher, you're stuck with that selection.
If you haven't yet selected a teacher here's how: After you've been in preschool for a while, you will get a special delivery mail [!] that takes you to the school to select your teacher. When you see the first teacher appear, press the A (left) button. That teacher will go away and a different one will appear instead. Press the A button again if that's still not the teacher you want. Press the B (middle) button when you see the teacher that you want. If a teacher appears, but you don't press any buttons for a while, you will get placed in that teachers class automatically.