Currently, the "login page" has a line that says "new user? sign up!" This allows anyone with that link to create an account. This means any of your clients can share the link, and anyone can access your courses if they hit "sign up".
My suggestion is you have a separate login page (where people can only get inside if they already are given access to one of your courses). And, you have a sign-up page for people to CREATE their login for your courses.
Ideally, I want to have a button on my website that says "Student Login" that is safe to be there without there being the "sign up" link in it.