If your school is rostered, your district takes care of your student and teacher accounts for you. You will not be able to add students to your pre-populated class manually. See How can I tell if my school is rostered and what does that mean? for more information.
You can add a new class by logging in to your account and clicking "Add" above the list of classes. Each of your classes needs a unique name to help you tell them apart.
Select the grade level of the class from the dropdown menu. This will be the starting grade for all students added to this class. However, you can change any individual student's grade to give them a different experience. You can see this article for instructions - How do I change the grade level for one of my students?
Rostered schools have classes that are created and populated when your school or district uploads files that contain student, teacher, class, and school information. However, teachers can still use the steps above to create custom classes in ST Math directly. This can be useful for differentiation, or for creating intervention groups. Teachers at rostered schools can link student accounts to these custom classes, but can not create new student accounts. Student accounts must be created through the school or district rostering system.