Auto Re-Enroll is designed to be used when rolling classes into a new time period. When you roll classes, all students can be rolled upon rolling classes or specific/selected students can be enrolled if auto re-enroll is checked for only those students.
The Auto Re-Enroll option is available at Student Details.
Note that if the class has fees associated with it, the student will be pending upon the re-enroll and must pay associated fees for the class.