The Candy Kingdom Preschool is located within the Candy Kingdom and is where kids are supposed to learn. The kids at the school are very misbehaved and lack proper discipline. The teacher is most likely a very old woman who cannot discipline them herself. Jake even had to grow big and yell to get the kids to listen to him. In the episode "Goliad," Finn makes an obstacle course (in an incredible short time) for Goliad to lead the children through.
It's just a regular building with a playground outside for the kids to play. Finn and Jake make an obstacle course for Goliad just near the playground area.