If you have bookmarked or saved the link to register your students, and your students are not able to create an account, you may be using an older version of the link that includes apps.corsava.com or sitecloud.corsava.com in part of the link. The most up-to-date link includes site.corsava.com and is always available on your New Student page by going to My Students, then + New Student and copying the link on the left side of the page. 

You can read more about registering your students using the link here