enum ClassType {A, B, C, D}

return DropdownButton(
    value: classType,
    onChanged: (ClassType newValue) {
      setState(() {
        viewModel.classType = newValue;
      });
    },
    items: ClassType.values.map((ClassType classType) {
      return DropdownMenuItem(
        value: classType,
        child: Text(classType.toString()));
    }).toList();
);