There are several ways to do it that depend a bit on how your school is choosing to set things up. Here is a basic guide to enrolling users in a course: https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-add-users-to-a-course/ta-p/1119

If you are connecting Canvas to an SIS like PowerSchool, that integration helps provide a way to enroll students using their enrollment in a course in PowerSchool. I have never done this before, so maybe someone else can give some insight to that process.