On your 'My Classes' page, look under your name in the upper right corner. You'll see if you're 'rostered', or 'non-rostered.'
There are multiple ways to get student data into ST Math and the method depends on your school or district. It can be uploaded by your district, from a third party service provider, like Clever or ClassLink, or teachers and admins can enter it in manually.
A school is considered rostered if the data is coming from the school’s student information system either through imports directly into ST Math or through a third party, like Clever or ClassLink. That means that you won't have to add new teacher or student accounts, or create classes - they'll take care of that for you. However, you can create classes that include students who are rostered in your school. See: How do I create a staff-managed class and add students?
A school is non-rostered if teachers create and manage the student accounts themselves. See: How do I create a class and add students?