AddCoursesToStudents.cs

var context = new StudentSystemContext();
var allStudents = context.Students;

foreach (var student in allStudents)
{
    Random randomCourse = new Random();
    student.Courses.Add(context.Courses.Find(
        randomCourse.Next(1, context.Courses.Count() + 1)
    ));
}
context.SaveChanges();