You'll get a [!] letter that takes you the preschool level soon after your tam evolves into the toddler. You'll get another that takes you to elementary school soon after you turn into a teenager. After you've gotten the letter, go to the connection icon (the heart, top row, 5th icon) and select "work." This will take you to school whenever you please.
When you get the letter for elementary school, you will be able to scroll between 3 different teachers by pressing A. Press B to select one.
Mr Turtle teaches Intelligence (The Pen)
Mr Canvas teaches social (the flower)
Ms Flowers teaches arts/fashion (the star)
After that, that teacher will be your teacher until you evolve into an adult, where you will graduate and get a job within 24 hours of becoming an adult.