If you have the appropriate privileges and need to make changes to your own Class Administrator account follow the steps below. The main tasks that need to be completed for you to use Track My Progress with your new students is to un-assign last year's students from your class and to assign your new students to your class. You will need to determine if this process will be taken care of for you by a School Administrator or District Administrator or whether you will need to do this yourself. If you have confirmed with your administrator that this process will be handled centrally for the whole school or district the only thing you will need to do is to confirm that the roster of students assigned to your Track My Progress account matches what you expect.
Follow the steps in http://support.trackmyprogress.com/knowledge/user-accounts-classes#student-accounts to un-assign last year's students from your class. These students will not be deleted from your school's Track My Progress account. They will only be removed from your account.
Follow the steps in How can I create a new Student Account from my Class Administrator account? to add your new students to your class.