When students are using my course access code to sign up for an account, they're being asked to provide an email address. Why? I thought students could create accounts with either a username or an email address?
In most cases your students can create accounts using either a username or email address - except in cases where you've created a course before adding a school to your account. When your teacher account and your course don't have a school associated with them, your students will need to use their email addresses to create accounts. This ensures that the student can reset their password in case they forget it. Once you've selected your school or added your school to Schoology, students will be able to create accounts with usernames.