using System;

using System.Linq;

public class Program

{

    public class School

    {

        public char Class { get; set; }

        public Activity Activity { get; set; }

        public char Groups { get; set; }

    }

    public enum Activity

    {

        Undefined,

        Dance,

        Sing

    }

    public static void Main()

    {

        School[] schoolList =

        {

            new() { Class = 'A', Activity = Activity.Dance, Groups = 'B'},

            new() { Class = 'A', Activity = Activity.Sing, Groups = 'B' },

            new() { Class = 'A', Activity = Activity.Sing, Groups = 'B' },

            new() { Class = 'B', Activity = Activity.Dance, Groups = 'B' },

            new() { Class = 'B', Activity = Activity.Sing, Groups = 'B' }

        };

        var result = schoolList

            .GroupBy(school => new { school.Class, school.Activity })

            .Select(gr => gr.First())

            .GroupBy(row => row.Activity)

            .All(gr => gr.Select(row => row.Groups).Distinct().Count() == 1);

            //.All(gr => !gr.Select(row => row.Groups).Distinct().Skip(1).Any());

        Console.WriteLine(result);

    }

}