Take a quick gander in map view or edit town to see if there are any other families with school-aged children. It's possible there aren't any; story progression will take steps to remedy that eventually, but it's not going to happen right away necessarily. Or you can try dropping an extra family with kids into the town. Give the kids a day to show up at school and see if your active child can interact. If not, you know you've got a problem bigger than just no other kids in town.