There is no letter and no official warning or anything of that nature when you have to go to preschool. There is also no School icon.
However, the last icon in the top row is the Door Icon, and inside of it you hace the options AWAY, PC, and OTHER. The PC is the log in code for your computer is the Music City Tama Town, and Other is to connect to other Tamagotchi's (Actually, I believe that it says something else, not OTHER and I can't find my Music Star right now, or I would make sure.)
By selecting AWAY as aa child, you will automatically go to Preschool, and there you can dance and play jump rope with other child Tama's.