In learning management system, how should students get invited or join classes from teachers?

I have an important question regarding the structure of users in accessing data from another user. I am creating a web app Learning Management System which is an offline application.

I need to know what is the best way to approach on how should Students join classes from Teachers to get their quizzes and exams ready.

I already have plans to construct these:

1. Get a code that will be create by teachers

2. On the register form input a code-ready that is made by the teacher.

3. Get invited by the teacher

Although some says make an Add Friend module but I think that wouldn't make the best of the structure.

But again, what is the best structure to do this?