No sorting required for this exercise, just print out the data as received =>

;-)

using System;
using System.Collections.Generic;
using System.Linq;
namespace _06.Courses
{
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<string, List<string>> studentByCourse = new Dictionary<string, List<string>>();
            while (true)
            {
                string line = Console.ReadLine();
                if (line == "end")
                {
                    break;
                }
                string[] parts = line.Split(" : ", StringSplitOptions.RemoveEmptyEntries)
                    .ToArray();

                string course = parts[0];
                string student = parts[1];

                if (!studentByCourse.ContainsKey(course))
                {
                    studentByCourse.Add(course, new List<string>() {student});
                }
                else
                {
                    studentByCourse[course].Add(student);
                }
            }
            // foreach (var course in studentByCourse.OrderByDescending(x => x.Value.Count()))
            foreach (var course in studentByCourse)
            {
               Console.WriteLine($"{course.Key}: {course.Value.Count()}");
            //    foreach (var name in course.Value.OrderBy(x =>x))
               foreach (var name in course.Value)
               {
                   Console.WriteLine($"-- {name}");
               }
            }
            // Dictionary<string, List<string>> sortedCourse = studentByCourse
            //     .OrderByDescending(c => c.Value.Count)
            //     .ToDictionary(x => x.Key, x => x.Value);

            // foreach (var kvp in sortedCourse)
            // {
            //     Console.WriteLine($"{kvp.Key}: {kvp.Value.Count}");
            //     List<string> sortedStudents = kvp.Value
            //         .OrderByDescending(x => x)
            //         .ToList();
            //     //course.Value.Sort();
            //     foreach (var student in sortedStudents)
            //     {
            //         Console.WriteLine($"-- {student}");
            //     }
            // }
        }
    }
}