Make your courses and then in the course administration settings, go to Users>enrolment methods and choose "self enrolment " When you go into the settings you can change the "default assigned role" from student to teacher - and if you then add an enrolment key which you give to your professors then they can enrol themselves as teachers onto their courses with the enrolment key you gave them.
Security and privacy
Professor and student self-enrollment help.
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