teach=Int32.Parse(Console.ReadLine());
String[] arrTeach = new String[teach]
for (Int32 i = 0; i < teach; i++)
            arrTeach[i] = Console.ReadLine();

//It no worked correct on timus compiler! Why?

Console.ReadLine() return string
You should just parse to int

 for (long i = 0; i < M; i++)
     {
       arrS[i] = long.Parse(Console.ReadLine());
    }

in your case arrTeach[i] = int.Parse(Console.ReadLine());
good luck with C# :)
Sorry for my English