if you want to get a different teacher you need to press the A button when you first see your teacher, I think now you've "chosen" Mr Turtlepedia you just have to put up with him 'till your next generation
When the teachers arrive, you will see "Mr. Turtlepedia".
Press the A button to scroll through teachers. To select the one you desire to have as a teacher, press the "B" button. As TamaMum said, if you keep that certain teacher on the screen too long, that will become your teacher..