Are you talking the preschool on Tamatown or in the V4? Anyway you have to wait until your Tama gets an invitation I think one of those [!] in the mail make sure you go to that, hit B when it gets above your head and then B again when you see the away message. Then you should see your Tama going to the school and see the teacher. From then on you can go to school.
I think this takes a bit to happen so don't worry, just watch the mail.