Avoid injecting Service/Repository in an Entity

Hi Everyone, 

 From what I've read, this is really bad. But I have the following use case so wondering how to do this in the proper DDD way. The business rule is contrived but it is very similar to what i have in real life. I just cannot disclose the use case due to NDAs.
 

 I have a class named Class. A class can have students in it. There is a business rule such that if a student with the same surname belongs to any current of any other class, they cannot be added to the class.
 

 So on Class, i have a method named AddStudent. Just wondering how would the algorithm be implemented in a proper DDD sense
 

 public class Class
 {
 public void AddStudent(Student student)
 {

 // Call a repository to get all the students enrolled in all classes
 // If any students surname matches the surname of the student passed in, do not thing
 // otherwise add the student

 }

 }