SimTown Elementary is a school for Preteen Sims to go to at any time of the day. It is unlocked once the Preparing For Preteens quest has been completed.
Preteens can only earn pocket money (Simoleons) by going to school. Their leveling system works with grades. Students attend school for 7 hours for levels 1-6 and 5 hours for levels 7 and 8.
Building SimTown Elementary also unlocks the Teacher career for adults. If a sim is at Level 1-3 on the career, then they will teach for 7 hours. But if they are at level 4 or 5, then they teach for 5 hours.