I think it’s more elegant to have

List neededCourses =
(from courseObject in students.Select(stu => stu.Value.Courses)
from individualCRN in courseObject
select individualCRN).Distinct().OrderBy(crn => crn).ToList();

than to have

var allCourses = students.Select(x => x.Value.Courses);
List neededCourses = new List();
foreach (var crse in allCourses)
{
foreach (int crn in crse)
neededCourses.Add(crn);
}
neededCourses = neededCourses.Distinct().OrderBy(x => x).ToList();

So I just wanted to say thanks!